Quadcopter Hover Tuner

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
<title>Quadcopter Hover Tuner — sloprun.dev</title>
<style>
/* ── sloprun design tokens (verbatim from demos/tokens.css) ───────────────── */
:root {
  --bg: #F6F6F3;          /* neutral paper, not cream */
  --surface: #FFFFFF;
  --ink: #1A1D21;
  --muted: #5A6068;
  --line: #E3E2DC;
  --accent: #C05600;      /* phosphor amber, darkened for light bg */
  --accent-soft: #FFF3E6;
  --run: #1A7F37;         /* earned green */
  --run-soft: #E7F4EA;
  --danger: #C93C37;
  --radius: 10px;
  --font-sans: system-ui, -apple-system, "Segoe UI", sans-serif;
  --font-mono: ui-monospace, "SF Mono", "Cascadia Code", Menlo, Consolas, monospace;
}
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) {
    --bg: #14161A; --surface: #1C1F26; --ink: #E8E6E1; --muted: #9AA0A8;
    --line: #2A2E36; --accent: #FFB454; --accent-soft: #2A2118;
    --run: #3FB950; --run-soft: #16281B; --danger: #F47067;
  }
}
:root[data-theme="dark"] {
  --bg: #14161A; --surface: #1C1F26; --ink: #E8E6E1; --muted: #9AA0A8;
  --line: #2A2E36; --accent: #FFB454; --accent-soft: #2A2118;
  --run: #3FB950; --run-soft: #16281B; --danger: #F47067;
}
body { background: var(--bg); color: var(--ink); font-family: var(--font-sans); }
/* ── end tokens ───────────────────────────────────────────────────────────── */

/* demo-local additions (scrims need alpha; tokens are opaque hex) */
:root { --scrim: rgba(246,246,243,.88); --shadow: 0 1px 2px rgba(26,29,33,.05); }
@media (prefers-color-scheme: dark) {
  :root:not([data-theme="light"]) { --scrim: rgba(20,22,26,.9); --shadow: 0 1px 2px rgba(0,0,0,.35); }
}
:root[data-theme="dark"] { --scrim: rgba(20,22,26,.9); --shadow: 0 1px 2px rgba(0,0,0,.35); }

* { box-sizing: border-box; }
html, body { margin: 0; padding: 0; }
html { -webkit-text-size-adjust: 100%; }
body { min-height: 100%; overflow-x: hidden; }

.wrap {
  max-width: 1140px; margin: 0 auto;
  padding: 22px 16px 26px;
  display: flex; flex-direction: column; gap: 16px;
}

h1 { margin: 0 0 6px; font-size: clamp(21px, 3.6vw, 29px); letter-spacing: -.021em; line-height: 1.15; }
.lede { margin: 0; max-width: 68ch; color: var(--muted); font-size: 14.5px; line-height: 1.55; }
.lede b { color: var(--ink); font-weight: 600; }

.stage { display: grid; grid-template-columns: minmax(0,1fr) 300px; gap: 16px; align-items: start; }
@media (max-width: 900px) {
  .stage { grid-template-columns: minmax(0,1fr); }
  /* gust is the fun bit — do not bury it under three cards on a phone */
  .panel > #ctlCard { order: -1; }
}
.left { display: flex; flex-direction: column; gap: 12px; min-width: 0; }
.panel { display: flex; flex-direction: column; gap: 12px; min-width: 0; }

.card {
  background: var(--surface); border: 1px solid var(--line);
  border-radius: var(--radius); padding: 12px 13px; box-shadow: var(--shadow);
}
.card h2 {
  margin: 0 0 10px; font-family: var(--font-mono); font-size: 10.5px;
  letter-spacing: .1em; text-transform: uppercase; color: var(--muted); font-weight: 600;
}

/* the flight scene */
.field {
  position: relative; overflow: hidden;
  border: 1px solid var(--line); border-radius: var(--radius);
  background-color: var(--surface);
  background-image: radial-gradient(var(--line) 1.1px, transparent 1.2px);
  background-size: 30px 30px; background-position: -15px -15px;
  aspect-ratio: 16 / 10; box-shadow: var(--shadow);
}
@media (max-width: 900px) { .field { aspect-ratio: 4 / 3; } }
@media (max-width: 520px) { .field { aspect-ratio: 1 / 1; } }
canvas { display: block; width: 100%; height: 100%; touch-action: manipulation; }
#scene { cursor: crosshair; }
canvas:focus { outline: none; }
canvas:focus-visible { outline: 2px solid var(--accent); outline-offset: -3px; }

.hud {
  position: absolute; left: 10px; top: 10px; display: flex; flex-wrap: wrap; gap: 4px 6px;
  pointer-events: none; max-width: calc(100% - 20px);
}
.hud span {
  font-family: var(--font-mono); font-size: 10.5px; letter-spacing: .04em;
  color: var(--muted); background: var(--scrim);
  border: 1px solid var(--line); border-radius: 999px; padding: 3px 8px;
  font-variant-numeric: tabular-nums; white-space: nowrap;
}
.hud span b { color: var(--ink); font-weight: 600; }

.hint {
  position: absolute; right: 10px; bottom: 10px; pointer-events: none;
  font-family: var(--font-mono); font-size: 11px; letter-spacing: .01em;
  color: var(--muted); background: var(--scrim);
  border: 1px solid var(--line); border-radius: 999px; padding: 5px 10px;
  transition: opacity .45s ease; max-width: calc(100% - 20px);
}
.hint[hidden] { display: none; }
.hint.gone { opacity: 0; }
.hint .h-narrow { display: none; }
@media (max-width: 560px) {
  /* the long version ran over the GROUND label on a phone */
  .hint { font-size: 10px; padding: 4px 9px; }
  .hint .h-wide { display: none; }
  .hint .h-narrow { display: inline; }
}

/* the paused card sits at the foot of the scene instead of covering it, so a
   reduced-motion visitor still sees the quad, the line and the ruler at once */
.overlay { position: absolute; inset: auto 10px 10px; display: flex; justify-content: center; pointer-events: none; }
.overlay[hidden] { display: none; }
.overlay .box {
  pointer-events: auto; display: flex; align-items: center; gap: 10px; text-align: left;
  max-width: 100%; background: var(--scrim); border: 1px solid var(--line);
  border-radius: var(--radius); padding: 8px 10px; box-shadow: var(--shadow);
}
.overlay .box button { flex: none; }
.overlay p { margin: 0; font-size: 11.5px; line-height: 1.4; color: var(--muted); }
@media (max-width: 560px) { .overlay .box { flex-direction: column; align-items: stretch; gap: 7px; } }

/* response plot */
.plotwrap { position: relative; height: 152px; }
@media (max-width: 520px) { .plotwrap { height: 124px; } }
.plotwrap canvas { border-radius: 6px; }
.legend {
  display: flex; flex-wrap: wrap; gap: 4px 14px; margin-top: 8px;
  font-family: var(--font-mono); font-size: 10px; letter-spacing: .06em;
  text-transform: uppercase; color: var(--muted);
}
.legend i { display: inline-block; width: 14px; height: 0; vertical-align: middle;
  border-top: 2px solid var(--muted); margin-right: 6px; }
.legend .alt i { border-top-color: var(--accent); }
.legend .set i { border-top-style: dashed; }
.legend .gst i { border-top-color: var(--danger); }

/* metrics */
.stats { display: grid; grid-template-columns: repeat(3, minmax(0,1fr)); gap: 10px; }
.stat { display: flex; flex-direction: column; gap: 2px; min-width: 0; }
.stat .v {
  font-family: var(--font-mono); font-size: clamp(16px, 4.2vw, 20px); line-height: 1.05;
  font-variant-numeric: tabular-nums; letter-spacing: -.02em; white-space: nowrap;
}
.stat .v .u { font-size: 11px; color: var(--muted); margin-left: 2px; letter-spacing: 0; }
.stat .k {
  font-family: var(--font-mono); font-size: 9.5px; letter-spacing: .09em;
  text-transform: uppercase; color: var(--muted);
}
.stat .v.warn { color: var(--accent); }
.stat .v.bad  { color: var(--danger); }
.stat .v.good { color: var(--run); }

.pill {
  display: inline-flex; align-items: center; gap: 6px; margin-top: 11px;
  font-family: var(--font-mono); font-size: 11px; letter-spacing: .04em;
  padding: 3px 9px 3px 7px; border-radius: 999px;
  border: 1px solid var(--line); color: var(--muted); background: var(--bg);
}
.pill .dot { width: 7px; height: 7px; border-radius: 50%; background: var(--muted); flex: none; }
.pill.on { color: var(--run); background: var(--run-soft); border-color: transparent; }
.pill.on .dot { background: var(--run); }
.pill.work { color: var(--accent); background: var(--accent-soft); border-color: transparent; }
.pill.work .dot { background: var(--accent); }
.pill.bad { color: var(--danger); border-color: var(--danger); background: var(--surface); }
.pill.bad .dot { background: var(--danger); }

.cap { margin: 9px 0 0; font-size: 11.5px; line-height: 1.45; color: var(--muted); }
.cap b { color: var(--ink); font-weight: 600; }

/* controls */
.ctl { display: block; margin-bottom: 14px; }
.ctl:last-child { margin-bottom: 2px; }
.ctl-top { display: flex; justify-content: space-between; align-items: baseline; gap: 8px; margin-bottom: 3px; }
.ctl-top label { font-size: 13px; }
.ctl-top label .g { font-family: var(--font-mono); color: var(--accent); font-size: 12px; margin-right: 5px; }
.ctl-top .val {
  font-family: var(--font-mono); font-size: 12px; color: var(--muted);
  font-variant-numeric: tabular-nums;
}
.ctl .desc { display: block; font-size: 11.5px; color: var(--muted); line-height: 1.4; margin-top: 3px; }

input[type=range] {
  -webkit-appearance: none; appearance: none; display: block;
  width: 100%; height: 20px; margin: 0; background: transparent; cursor: pointer;
}
input[type=range]::-webkit-slider-runnable-track { height: 4px; border-radius: 2px; background: var(--line); }
input[type=range]::-webkit-slider-thumb {
  -webkit-appearance: none; width: 15px; height: 15px; margin-top: -5.5px;
  border-radius: 50%; background: var(--accent); border: 2px solid var(--surface);
  box-shadow: 0 0 0 1px var(--line);
}
input[type=range]::-moz-range-track { height: 4px; border-radius: 2px; background: var(--line); }
input[type=range]::-moz-range-thumb {
  width: 13px; height: 13px; border-radius: 50%; background: var(--accent);
  border: 2px solid var(--surface); box-shadow: 0 0 0 1px var(--line);
}
input[type=range]:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; border-radius: 6px; }

button {
  font: inherit; font-size: 13px; font-family: var(--font-sans);
  color: var(--ink); background: var(--surface);
  border: 1px solid var(--line); border-radius: 8px;
  padding: 7px 11px; cursor: pointer; line-height: 1.2;
  transition: border-color .12s ease, color .12s ease, background .12s ease;
}
button:hover:not(:disabled) { border-color: var(--accent); color: var(--accent); }
button:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }
button:disabled { opacity: .42; cursor: not-allowed; }
button.primary { background: var(--accent); color: var(--surface); border-color: var(--accent); font-weight: 600; }
button.primary:hover:not(:disabled) { color: var(--surface); filter: brightness(1.07); }
button.big { font-size: 14px; padding: 9px 18px; }

.row { display: flex; flex-wrap: wrap; gap: 7px; }
.row > button { flex: 1 1 auto; }
.chips { display: flex; flex-wrap: wrap; gap: 7px; }
.chips button { flex: 1 1 auto; font-family: var(--font-mono); font-size: 11.5px; letter-spacing: .02em; padding: 6px 9px; }
.chips button[aria-pressed="true"] {
  background: var(--accent-soft); color: var(--accent); border-color: var(--accent);
}

footer {
  font-family: var(--font-mono); font-size: 11px; color: var(--muted);
  letter-spacing: .05em; padding-top: 2px;
}

.sr-only {
  position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px;
  overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; border: 0;
}

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { transition-duration: .001ms !important; animation-duration: .001ms !important; }
}
</style>
</head>
<body>
<div class="wrap">

  <header>
    <h1>Quadcopter Hover Tuner</h1>
    <p class="lede">
      A hobby quadcopter cannot simply "hold 6 metres" — something has to decide, hundreds of times
      a second, how much throttle to give the props. That something is a <b>PID controller</b>, and it
      has exactly three knobs. Tune them, <b>click the sky</b> to ask for a different altitude, and hit
      <b>gust</b> to shove the quad off its perch.
    </p>
  </header>

  <main class="stage">

    <div class="left">

      <div class="field" id="field">
        <canvas id="scene" tabindex="0" aria-label="Side view of a hovering quadcopter. Click, or press the up and down arrow keys, to change the altitude it is asked to hold. Press Enter or Space for a wind gust.">
          A side-view animation of a quadcopter holding altitude. Your browser does not support canvas.
        </canvas>
        <div class="hud" aria-hidden="true">
          <span>ALT <b id="hAlt">6.00</b> m</span>
          <span>V/S <b id="hVs">+0.00</b> m/s</span>
          <span>THR <b id="hThr">38</b> %</span>
        </div>
        <div class="hint" id="hint"><span class="h-wide">click the sky to change the altitude</span><span class="h-narrow">tap the sky to set the altitude</span></div>
        <div class="overlay" id="overlay" hidden>
          <div class="box">
            <button class="primary big" id="overlayPlay" type="button">▶&nbsp; Fly it</button>
            <p id="overlayNote">Paused. Every control still works.</p>
          </div>
        </div>
      </div>

      <section class="card" aria-label="Altitude response over time">
        <h2>Response — last 12 seconds</h2>
        <div class="plotwrap">
          <canvas id="plot" role="img" aria-label="Line chart of altitude against time with the requested altitude drawn as a dashed line.">
            A chart of altitude over time.
          </canvas>
        </div>
        <div class="legend">
          <span class="alt"><i></i>altitude</span>
          <span class="set"><i></i>asked for</span>
          <span class="gst"><i></i>gust</span>
        </div>
      </section>

    </div>

    <div class="panel">

      <section class="card" aria-label="Hold quality">
        <h2>Hold quality</h2>
        <div class="stats">
          <div class="stat"><span class="v" id="sOver">—</span><span class="k">overshoot</span></div>
          <div class="stat"><span class="v" id="sSettle">—</span><span class="k">settle time</span></div>
          <div class="stat"><span class="v" id="sErr">—</span><span class="k">hold error</span></div>
        </div>
        <span class="pill" id="pill"><span class="dot"></span><span id="pillText">idle</span></span>
        <p class="cap">Measured from the last test step, altitude change or gust. <b>Settled</b> means it stayed inside the band around the line for a full second.</p>
      </section>

      <section class="card" aria-label="Controller gains">
        <h2>The three knobs</h2>

        <div class="ctl">
          <div class="ctl-top">
            <label for="kp"><span class="g">Kp</span> Push</label><span class="val" id="kpV" aria-hidden="true">16.0</span>
          </div>
          <input type="range" id="kp" min="0" max="30" step="0.5" value="16" aria-describedby="kpD">
          <span class="desc" id="kpD">Reacts to the gap right now. Too little and it hangs below the line; too much and it charges straight past it.</span>
        </div>

        <div class="ctl">
          <div class="ctl-top">
            <label for="ki"><span class="g">Ki</span> Memory</label><span class="val" id="kiV" aria-hidden="true">6.00</span>
          </div>
          <input type="range" id="ki" min="0" max="15" step="0.25" value="6" aria-describedby="kiD">
          <span class="desc" id="kiD">Notices a gap that never closes and quietly adds throttle. Drag it to zero and watch the quad sag below the line forever.</span>
        </div>

        <div class="ctl">
          <div class="ctl-top">
            <label for="kd"><span class="g">Kd</span> Brakes</label><span class="val" id="kdV" aria-hidden="true">3.50</span>
          </div>
          <input type="range" id="kd" min="0" max="12" step="0.1" value="3.5" aria-describedby="kdD">
          <span class="desc" id="kdD">Watches how fast the gap is closing and eases off early. Damps the bounce — too much and the throttle gets fidgety.</span>
        </div>
      </section>

      <section class="card" aria-label="Presets">
        <h2>Try a tune</h2>
        <div class="chips" id="presets" role="group" aria-label="Gain presets">
          <button type="button" data-p="0" aria-pressed="false">sluggish</button>
          <button type="button" data-p="1" aria-pressed="false">twitchy</button>
          <button type="button" data-p="2" aria-pressed="false">well&nbsp;tuned</button>
        </div>
        <p class="cap" id="presetNote">Loaded with a middling tune — it holds the line, but it bounces on the way. Every tune flies the <b>same</b> 3 → 6 m test step, so the three numbers above compare like for like.</p>
      </section>

      <section class="card" id="ctlCard" aria-label="Flight controls">
        <h2>Controls</h2>
        <div class="row" style="margin-bottom:7px">
          <button type="button" class="primary" id="gustBtn">Gust</button>
          <button type="button" id="takeoffBtn">Land &amp; take off</button>
        </div>
        <div class="row">
          <button type="button" id="playBtn">Pause</button>
          <button type="button" id="upBtn" title="Re-fly the standard 3 to 6 metre step with the current gains">Test step</button>
        </div>
      </section>

    </div>
  </main>

  <footer>demo · sloprun.dev</footer>
</div>

<div class="sr-only" role="status" aria-live="polite" id="live"></div>

<script>
(function () {
  "use strict";

  var root = document.documentElement;
  var mqDark = window.matchMedia ? matchMedia("(prefers-color-scheme: dark)") : null;
  var mqRM   = window.matchMedia ? matchMedia("(prefers-reduced-motion: reduce)") : null;
  var reduced = !!(mqRM && mqRM.matches);

  /* ── dom ────────────────────────────────────────────────────────────────── */
  var field   = document.getElementById("field");
  var scene   = document.getElementById("scene");
  var plot    = document.getElementById("plot");
  var sctx    = scene.getContext ? scene.getContext("2d") : null;
  var pctx    = plot.getContext ? plot.getContext("2d") : null;
  if (!sctx || !pctx) return;

  var hAlt = document.getElementById("hAlt");
  var hVs  = document.getElementById("hVs");
  var hThr = document.getElementById("hThr");
  var hint = document.getElementById("hint");
  var overlay = document.getElementById("overlay");
  var overlayPlay = document.getElementById("overlayPlay");
  var overlayNote = document.getElementById("overlayNote");
  var sOver = document.getElementById("sOver");
  var sSettle = document.getElementById("sSettle");
  var sErr = document.getElementById("sErr");
  var pill = document.getElementById("pill");
  var pillText = document.getElementById("pillText");
  var live = document.getElementById("live");
  var presetsEl = document.getElementById("presets");
  var presetNote = document.getElementById("presetNote");
  var gustBtn = document.getElementById("gustBtn");
  var takeoffBtn = document.getElementById("takeoffBtn");
  var playBtn = document.getElementById("playBtn");
  var upBtn = document.getElementById("upBtn");
  var sliders = { kp: document.getElementById("kp"), ki: document.getElementById("ki"), kd: document.getElementById("kd") };
  var vals    = { kp: document.getElementById("kpV"), ki: document.getElementById("kiV"), kd: document.getElementById("kdV") };

  /* ── colours read from the tokens, so both themes just work ─────────────── */
  var COL = {
    surface: [255,255,255], bg: [246,246,243], ink: [26,29,33], muted: [90,96,104],
    line: [227,226,220], accent: [192,86,0], run: [26,127,55], danger: [201,60,55]
  };
  function hexToRgb(h) {
    h = String(h).trim();
    if (h.charAt(0) === "#") h = h.slice(1);
    if (h.length === 3) h = h.charAt(0)+h.charAt(0)+h.charAt(1)+h.charAt(1)+h.charAt(2)+h.charAt(2);
    var n = parseInt(h, 16);
    if (h.length !== 6 || isNaN(n)) return null;
    return [(n >> 16) & 255, (n >> 8) & 255, n & 255];
  }
  function tok(name, fb) {
    var v = getComputedStyle(root).getPropertyValue(name);
    return hexToRgb(v) || fb;
  }
  function rgba(c, a) { return "rgba(" + c[0] + "," + c[1] + "," + c[2] + "," + a + ")"; }
  function readTheme() {
    COL.surface = tok("--surface", COL.surface);
    COL.bg      = tok("--bg", COL.bg);
    COL.ink     = tok("--ink", COL.ink);
    COL.muted   = tok("--muted", COL.muted);
    COL.line    = tok("--line", COL.line);
    COL.accent  = tok("--accent", COL.accent);
    COL.run     = tok("--run", COL.run);
    COL.danger  = tok("--danger", COL.danger);
  }
  var MONO = 'ui-monospace, "SF Mono", Menlo, Consolas, monospace';

  window.addEventListener("message", function (e) {
    var d = e.data;
    if (d && d.type === "sloprun:theme" && (d.theme === "light" || d.theme === "dark")) {
      root.setAttribute("data-theme", d.theme);
      readTheme(); render();
    }
  });
  if (mqDark) {
    var onScheme = function () { readTheme(); render(); };
    if (mqDark.addEventListener) mqDark.addEventListener("change", onScheme);
    else if (mqDark.addListener) mqDark.addListener(onScheme);
  }

  /* ── flight model ───────────────────────────────────────────────────────────
     One axis only: altitude. Thrust points up, gravity pulls down, air drags.
     The controller may command 0 .. U_MAX of upward acceleration; hovering needs
     exactly G of it, which is the whole reason the "memory" knob exists.        */
  var G = 9.81;              /* m/s^2 */
  var DRAG = 0.35;           /* 1/s, linear air drag on climb rate */
  var U_MAX = 26;            /* m/s^2 at full throttle -> thrust-to-weight ~2.6 */
  var ALT_MAX = 11.0;        /* ceiling of the scene, metres */
  var ALT_VIEW = 11.0;
  var SUB = 1 / 300;         /* physics step */
  var SAMPLE = 1 / 60;       /* history sample rate */
  var WINDOW = 12;           /* seconds of plot */

  var y = 0, v = 0, I = 0, u = 0;
  var sp = 3.0;
  var simT = 0;
  var Kp = 16, Ki = 6, Kd = 3.5;

  var gustT0 = -99, gustDur = 0.42, gustAmp = 0;
  var sway = 0, swayV = 0, spin = 0;

  /* ── rolling history (ring buffer) ──────────────────────────────────────── */
  var CAP = 1024;
  var hT = new Float32Array(CAP), hY = new Float32Array(CAP), hS = new Float32Array(CAP);
  var hHead = 0, hLen = 0, sampleAcc = 0;
  var gusts = [];

  function pushSample(t, alt, set) {
    var i;
    if (hLen < CAP) { i = (hHead + hLen) % CAP; hLen++; }
    else { i = hHead; hHead = (hHead + 1) % CAP; }
    hT[i] = t; hY[i] = alt; hS[i] = set;
  }
  function clearHistory() { hHead = 0; hLen = 0; gusts.length = 0; sampleAcc = 0; }

  /* ── the episode: everything the metrics are measured against ───────────── */
  var ep = null, ssErr = 0;

  function newEpisode(kind) {
    ep = { t0: simT, y0: y, kind: kind, dev: 0, sgn: (y > sp ? 1 : -1),
           over: 0, lastExceed: simT, settled: false, settleT: 0 };
    ssErr = Math.abs(y - sp);
  }

  function updateMetrics(dt) {
    if (!ep) return;
    var e = y - sp;                       /* signed: above the line is positive */
    var ae = Math.abs(e);
    if (ae > ep.dev) { ep.dev = ae; ep.sgn = e >= 0 ? 1 : -1; }
    var band = Math.max(0.05, 0.02 * ep.dev);
    ep.band = band;
    if (ep.dev > 0.02) {
      var os = (-ep.sgn * e) / ep.dev;
      if (os > ep.over) ep.over = os;
    }
    if (ae > band) { ep.lastExceed = simT; ep.settled = false; }
    else if (!ep.settled && simT - ep.lastExceed >= 1.0) {
      ep.settled = true; ep.settleT = ep.lastExceed - ep.t0;
    }
    var k = Math.min(1, dt / 0.7);
    ssErr += (ae - ssErr) * k;
  }

  /* ── one physics substep ────────────────────────────────────────────────── */
  function substep(dt) {
    var e = sp - y;

    /* derivative acts on the measurement, not the error, so moving the line
       never produces a throttle spike */
    var raw = Kp * e + Ki * I - Kd * v;
    u = raw < 0 ? 0 : (raw > U_MAX ? U_MAX : raw);

    /* anti-windup: stop stuffing the integrator once the props are maxed out */
    var saturated = (raw > U_MAX && e > 0) || (raw < 0 && e < 0);
    if (!saturated) I += e * dt;
    var lim = Ki > 0.01 ? U_MAX / Ki : 0;
    if (I > lim) I = lim; else if (I < -lim) I = -lim;

    var g = 0;
    if (simT - gustT0 < gustDur) {
      g = gustAmp * Math.sin(Math.PI * (simT - gustT0) / gustDur);
    }

    var a = u - G - DRAG * v + g;
    v += a * dt;
    if (v > 40) v = 40; else if (v < -40) v = -40;
    y += v * dt;

    if (y <= 0) { y = 0; if (v < 0) v = 0; }
    if (y >= ALT_MAX) { y = ALT_MAX; if (v > 0) v = 0; }

    /* horizontal sway is decoration only — a gust shoves it sideways too */
    swayV += (g * 0.55) * dt;
    swayV += (-sway * 7.0 - swayV * 1.9) * dt;
    sway += swayV * dt;
    if (sway > 3) sway = 3; else if (sway < -3) sway = -3;

    spin += (0.6 + 2.4 * (u / U_MAX)) * 60 * dt;

    simT += dt;
    updateMetrics(dt);

    sampleAcc += dt;
    if (sampleAcc >= SAMPLE) { sampleAcc -= SAMPLE; pushSample(simT, y, sp); }
  }

  var acc = 0;
  function advance(dt) {
    acc += dt;
    var guard = 0;
    while (acc >= SUB && guard < 2000) { substep(SUB); acc -= SUB; guard++; }
    if (guard >= 2000) acc = 0;
  }

  /* ── canvas sizing ──────────────────────────────────────────────────────── */
  var SW = 0, SH = 0, PW = 0, PH = 0;

  function fit(cv, ctx2) {
    var rect = cv.getBoundingClientRect();
    var w = Math.max(80, Math.round(rect.width));
    var h = Math.max(60, Math.round(rect.height));
    var dpr = Math.min(2, window.devicePixelRatio || 1);
    var nw = Math.round(w * dpr), nh = Math.round(h * dpr);
    if (cv.width !== nw || cv.height !== nh) { cv.width = nw; cv.height = nh; }
    ctx2.setTransform(dpr, 0, 0, dpr, 0, 0);
    return [w, h];
  }
  function resize() {
    var a = fit(scene, sctx); SW = a[0]; SH = a[1];
    var b = fit(plot, pctx);  PW = b[0]; PH = b[1];
    render();
  }

  /* ── scene ──────────────────────────────────────────────────────────────── */
  function drawScene() {
    var w = SW, h = SH, ctx = sctx;
    if (!w || !h) return;
    ctx.clearRect(0, 0, w, h);

    var padT = 16, padB = 30, padL = w < 340 ? 34 : 38, padR = 14;
    var span = h - padT - padB;
    function ay(alt) { return h - padB - (alt / ALT_VIEW) * span; }

    /* ground */
    var gy = ay(0);
    ctx.fillStyle = rgba(COL.muted, 0.07);
    ctx.fillRect(0, gy, w, h - gy);
    ctx.strokeStyle = rgba(COL.muted, 0.45); ctx.lineWidth = 1;
    ctx.beginPath(); ctx.moveTo(0, gy + 0.5); ctx.lineTo(w, gy + 0.5); ctx.stroke();
    ctx.strokeStyle = rgba(COL.muted, 0.2);
    ctx.beginPath();
    for (var gx = -20; gx < w + 20; gx += 11) { ctx.moveTo(gx, h); ctx.lineTo(gx + 12, gy); }
    ctx.stroke();

    /* altitude ruler */
    ctx.font = "10px " + MONO;
    ctx.textBaseline = "middle"; ctx.textAlign = "right";
    for (var m = 0; m <= 10; m += 2) {
      var ty = ay(m);
      ctx.strokeStyle = rgba(COL.line, 0.9);
      ctx.beginPath(); ctx.moveTo(padL, ty + 0.5); ctx.lineTo(w - padR, ty + 0.5); ctx.stroke();
      ctx.fillStyle = rgba(COL.muted, 0.95);
      ctx.fillText(m === 10 ? "10 m" : String(m), padL - 8, ty);
    }

    /* the band we call "settled", then the line itself */
    var band = ep && ep.band ? ep.band : 0.05;
    var bt = ay(sp + band), bb = ay(sp - band);
    var holding = !!(ep && ep.settled);
    if (bb - bt >= 1.5) {
      ctx.fillStyle = holding ? rgba(COL.run, 0.13) : rgba(COL.accent, 0.08);
      ctx.fillRect(padL, bt, w - padL - padR, bb - bt);
    }
    var sy = ay(sp);
    ctx.save();
    ctx.setLineDash([7, 5]);
    ctx.strokeStyle = holding ? rgba(COL.run, 0.95) : rgba(COL.accent, 0.95);
    ctx.lineWidth = 1.6;
    ctx.beginPath(); ctx.moveTo(padL, sy + 0.5); ctx.lineTo(w - padR, sy + 0.5); ctx.stroke();
    ctx.restore();

    /* label chip on the line */
    var lbl = "HOLD " + sp.toFixed(2) + " m";
    ctx.font = "10px " + MONO;
    var lw = ctx.measureText(lbl).width + 12;
    var lx = w - padR - lw, ly = sy - 9 < padT ? sy + 9 : sy - 9;
    ctx.fillStyle = rgba(COL.surface, 0.92);
    roundRect(ctx, lx, ly - 8, lw, 16, 8); ctx.fill();
    ctx.strokeStyle = holding ? rgba(COL.run, 0.5) : rgba(COL.accent, 0.45);
    ctx.lineWidth = 1; roundRect(ctx, lx, ly - 8, lw, 16, 8); ctx.stroke();
    ctx.fillStyle = holding ? rgba(COL.run, 1) : rgba(COL.accent, 1);
    ctx.textAlign = "center";
    ctx.fillText(lbl, lx + lw / 2, ly);

    /* the quad */
    var qx = w * 0.5 + sway * (w < 400 ? 5 : 8);
    var qy = ay(y);
    var s = Math.max(0.62, Math.min(1.25, w / 620));
    var tilt = Math.max(-0.28, Math.min(0.28, -swayV * 0.06));

    /* gap marker between craft and line */
    var e = y - sp;
    if (Math.abs(e) > 0.06) {
      ctx.save();
      ctx.setLineDash([2, 3]);
      ctx.strokeStyle = rgba(COL.muted, 0.75); ctx.lineWidth = 1;
      ctx.beginPath(); ctx.moveTo(qx, qy); ctx.lineTo(qx, sy); ctx.stroke();
      ctx.restore();
      if (Math.abs(e) > 0.3) {
        ctx.font = "10px " + MONO;
        ctx.textBaseline = "middle";
        var gapTxt = (e > 0 ? "+" : "−") + Math.abs(e).toFixed(2) + " m";
        var gw = ctx.measureText(gapTxt).width + 10;
        var off = 46 * s, gcx;
        if (qx + off + gw < w - padR) gcx = qx + off + gw / 2;
        else gcx = qx - off - gw / 2;
        var gcy = (qy + sy) / 2;
        ctx.fillStyle = rgba(COL.surface, 0.88);
        roundRect(ctx, gcx - gw / 2, gcy - 8, gw, 16, 8); ctx.fill();
        ctx.fillStyle = rgba(COL.muted, 1);
        ctx.textAlign = "center";
        ctx.fillText(gapTxt, gcx, gcy);
      }
    }

    /* downwash under each rotor, opacity tracks throttle */
    var thr = u / U_MAX;
    ctx.save();
    ctx.translate(qx, qy); ctx.rotate(tilt);
    var arm = 26 * s;
    for (var k = -1; k <= 1; k += 2) {
      var dx = k * arm;
      var gdt = ctx.createLinearGradient(0, -4 * s, 0, 46 * s);
      gdt.addColorStop(0, rgba(COL.accent, 0.20 * thr + 0.03));
      gdt.addColorStop(1, rgba(COL.accent, 0));
      ctx.fillStyle = gdt;
      ctx.beginPath();
      ctx.moveTo(dx - 9 * s, -2 * s); ctx.lineTo(dx + 9 * s, -2 * s);
      ctx.lineTo(dx + 15 * s, 46 * s); ctx.lineTo(dx - 15 * s, 46 * s);
      ctx.closePath(); ctx.fill();
    }

    /* legs */
    ctx.strokeStyle = rgba(COL.muted, 0.9); ctx.lineWidth = 1.6 * s;
    ctx.beginPath();
    ctx.moveTo(-9 * s, 5 * s); ctx.lineTo(-13 * s, 13 * s);
    ctx.moveTo(9 * s, 5 * s);  ctx.lineTo(13 * s, 13 * s);
    ctx.moveTo(-16 * s, 13 * s); ctx.lineTo(-10 * s, 13 * s);
    ctx.moveTo(16 * s, 13 * s);  ctx.lineTo(10 * s, 13 * s);
    ctx.stroke();

    /* arms + motors */
    ctx.strokeStyle = rgba(COL.ink, 0.85); ctx.lineWidth = 3 * s; ctx.lineCap = "round";
    ctx.beginPath();
    ctx.moveTo(-8 * s, -1 * s); ctx.lineTo(-arm, -7 * s);
    ctx.moveTo(8 * s, -1 * s);  ctx.lineTo(arm, -7 * s);
    ctx.stroke();
    ctx.lineCap = "butt";
    ctx.fillStyle = rgba(COL.ink, 0.85);
    for (k = -1; k <= 1; k += 2) ctx.fillRect(k * arm - 2.6 * s, -13 * s, 5.2 * s, 7 * s);

    /* body */
    ctx.fillStyle = rgba(COL.ink, 0.92);
    roundRect(ctx, -15 * s, -6 * s, 30 * s, 12 * s, 4 * s); ctx.fill();
    /* status light: amber while it works, green only once it is genuinely holding */
    ctx.fillStyle = holding ? rgba(COL.run, 1) : rgba(COL.accent, 1);
    ctx.beginPath(); ctx.arc(9 * s, 0, 2.2 * s, 0, 6.2832); ctx.fill();

    /* props — side on, so a spinning blade foreshortens to a moving sliver */
    var pr = 17 * s;
    for (k = -1; k <= 1; k += 2) {
      var mx = k * arm, my = -14 * s;
      ctx.strokeStyle = rgba(COL.accent, 0.28 + 0.35 * thr);
      ctx.lineWidth = 1.2 * s;
      ctx.beginPath(); ctx.ellipse(mx, my, pr, 2.6 * s, 0, 0, 6.2832); ctx.stroke();
      var f = reduced ? 0.85 : Math.abs(Math.cos(spin * (k > 0 ? 1 : -1.13)));
      ctx.strokeStyle = rgba(COL.ink, 0.75);
      ctx.lineWidth = 2 * s;
      ctx.beginPath(); ctx.moveTo(mx - pr * f, my); ctx.lineTo(mx + pr * f, my); ctx.stroke();
    }
    ctx.restore();

    /* gust streaks */
    var age = simT - gustT0;
    if (age >= 0 && age < gustDur + 0.35) {
      var fade = Math.max(0, 1 - age / (gustDur + 0.35));
      ctx.strokeStyle = rgba(COL.danger, 0.55 * fade);
      ctx.lineWidth = 1.4;
      var dir = gustAmp < 0 ? 1 : -1;
      for (var q = 0; q < 5; q++) {
        var ry = qy - 34 * s + q * 17 * s + (q % 2) * 5;
        var len = (26 + q * 9) * s;
        var ox = qx - 46 * s - q * 6 * s;
        ctx.beginPath();
        ctx.moveTo(ox, ry); ctx.lineTo(ox + len, ry + dir * 8 * s);
        ctx.stroke();
      }
      ctx.font = "10px " + MONO;
      ctx.textAlign = "right"; ctx.textBaseline = "top";
      ctx.fillStyle = rgba(COL.danger, fade);
      ctx.fillText(gustAmp < 0 ? "GUST ↓" : "GUST ↑", w - 12, padT - 6);
    }

    /* ground label */
    ctx.font = "9.5px " + MONO;
    ctx.textAlign = "left"; ctx.textBaseline = "top";
    ctx.fillStyle = rgba(COL.muted, 0.9);
    ctx.fillText("GROUND", padL, gy + 6);
  }

  function roundRect(ctx, x, yy, w, h, r) {
    var rr = Math.min(r, w / 2, h / 2);
    ctx.beginPath();
    ctx.moveTo(x + rr, yy);
    ctx.arcTo(x + w, yy, x + w, yy + h, rr);
    ctx.arcTo(x + w, yy + h, x, yy + h, rr);
    ctx.arcTo(x, yy + h, x, yy, rr);
    ctx.arcTo(x, yy, x + w, yy, rr);
    ctx.closePath();
  }

  /* ── response plot ──────────────────────────────────────────────────────────
     The vertical axis zooms to whatever the last twelve seconds actually did.
     A 21% overshoot on a 1.5 m step is 30 cm — invisible on a fixed 0–11 m
     axis, which is exactly the thing this plot exists to show. The scene
     canvas keeps the absolute ruler, so nothing is lost.                       */
  var pv0 = 0, pv1 = ALT_VIEW, pvSet = false;

  function plotRange() {
    var lo = sp, hi = sp, k, i, t0 = simT - WINDOW;
    if (y < lo) lo = y; if (y > hi) hi = y;
    for (k = 0; k < hLen; k++) {
      i = (hHead + k) % CAP;
      if (hT[i] < t0) continue;
      if (hY[i] < lo) lo = hY[i]; else if (hY[i] > hi) hi = hY[i];
      if (hS[i] < lo) lo = hS[i]; else if (hS[i] > hi) hi = hS[i];
    }
    var MIN = 1.0, mid;
    var pad = Math.max(0.08, (hi - lo) * 0.18);
    lo -= pad; hi += pad;
    if (hi - lo < MIN) { mid = (lo + hi) / 2; lo = mid - MIN / 2; hi = mid + MIN / 2; }
    if (lo < 0) { hi -= lo; lo = 0; }
    if (hi > ALT_MAX) { lo = Math.max(0, lo - (hi - ALT_MAX)); hi = ALT_MAX; }
    return [lo, hi];
  }

  /* smallest round step (1, 2 or 5 times a power of ten) that keeps the axis
     down to five gaps — so there are always three or four labelled lines */
  var STEPS = [1, 2, 5, 10];
  function niceStep(span) {
    var mag = Math.pow(10, Math.floor(Math.log(span / 4) / Math.LN10));
    for (var i = 0; i < STEPS.length; i++) {
      if (span / (STEPS[i] * mag) <= 5) return STEPS[i] * mag;
    }
    return 10 * mag;
  }

  function drawPlot() {
    var w = PW, h = PH, ctx = pctx;
    if (!w || !h) return;
    ctx.clearRect(0, 0, w, h);

    /* ease the axis so a new step slides into frame instead of snapping */
    var want = plotRange();
    if (!pvSet || reduced || !active) { pv0 = want[0]; pv1 = want[1]; pvSet = true; }
    else { pv0 += (want[0] - pv0) * 0.09; pv1 += (want[1] - pv1) * 0.09; }
    if (pv1 - pv0 < 0.2) pv1 = pv0 + 0.2;

    var padL = 40, padR = 8, padT = 8, padB = 16;
    var iw = w - padL - padR, ih = h - padT - padB;
    var t1 = simT, t0 = simT - WINDOW;
    function px(t) { return padL + (t - t0) / WINDOW * iw; }
    function py(alt) { return padT + ih - (alt - pv0) / (pv1 - pv0) * ih; }

    /* frame + gridlines */
    ctx.strokeStyle = rgba(COL.line, 1); ctx.lineWidth = 1;
    ctx.beginPath();
    ctx.moveTo(padL + .5, padT); ctx.lineTo(padL + .5, padT + ih + .5); ctx.lineTo(w - padR, padT + ih + .5);
    ctx.stroke();
    ctx.font = "9.5px " + MONO;
    ctx.textAlign = "right"; ctx.textBaseline = "middle";
    var step = niceStep(pv1 - pv0);
    var dec = step >= 1 ? 0 : (step >= 0.1 ? 1 : 2);
    var ticks = [];
    for (var m = Math.ceil(pv0 / step) * step; m <= pv1 + 1e-9; m += step) {
      var gy = py(m);
      if (gy < padT + 5 || gy > padT + ih - 1) continue;
      ticks.push([m, gy]);
    }
    for (var ti = 0; ti < ticks.length; ti++) {
      ctx.strokeStyle = rgba(COL.line, 0.85);
      ctx.beginPath(); ctx.moveTo(padL, ticks[ti][1] + .5); ctx.lineTo(w - padR, ticks[ti][1] + .5); ctx.stroke();
      ctx.fillStyle = rgba(COL.muted, 0.95);
      /* the topmost tick carries the unit, so the axis says it once */
      ctx.fillText(ticks[ti][0].toFixed(dec) + (ti === ticks.length - 1 ? " m" : ""), padL - 6, ticks[ti][1]);
    }
    ctx.textAlign = "center"; ctx.textBaseline = "top";
    for (var tt = 0; tt <= WINDOW; tt += 4) {
      var x = px(t0 + tt);
      ctx.fillStyle = rgba(COL.muted, 0.9);
      ctx.fillText(tt === WINDOW ? "now" : "-" + (WINDOW - tt) + "s", x, padT + ih + 4);
    }

    if (hLen < 2) return;
    ctx.save();
    ctx.beginPath(); ctx.rect(padL, padT, iw, ih); ctx.clip();

    /* settle band around the current line */
    var band = ep && ep.band ? ep.band : 0.05;
    var holding = !!(ep && ep.settled);
    var bt = py(sp + band), bb = py(sp - band);
    if (bb - bt < 2) { var mid = (bb + bt) / 2; bt = mid - 1; bb = mid + 1; }
    ctx.fillStyle = holding ? rgba(COL.run, 0.14) : rgba(COL.accent, 0.09);
    ctx.fillRect(padL, bt, iw, bb - bt);

    /* gust marks */
    for (var q = 0; q < gusts.length; q++) {
      if (gusts[q] < t0) continue;
      var gx = px(gusts[q]);
      ctx.strokeStyle = rgba(COL.danger, 0.55); ctx.lineWidth = 1.2;
      ctx.beginPath(); ctx.moveTo(gx, padT); ctx.lineTo(gx, padT + ih); ctx.stroke();
    }

    var i, k, first;

    /* what the pilot asked for (a staircase) */
    ctx.strokeStyle = rgba(COL.muted, 0.95); ctx.lineWidth = 1.3;
    ctx.save(); ctx.setLineDash([5, 4]);
    ctx.beginPath(); first = true;
    var prevS = 0;
    for (k = 0; k < hLen; k++) {
      i = (hHead + k) % CAP;
      if (hT[i] < t0) { prevS = hS[i]; continue; }
      var xx = px(hT[i]);
      if (first) { ctx.moveTo(xx, py(hS[i])); first = false; }
      else {
        if (hS[i] !== prevS) ctx.lineTo(xx, py(prevS));
        ctx.lineTo(xx, py(hS[i]));
      }
      prevS = hS[i];
    }
    if (!first) ctx.lineTo(px(t1), py(sp));
    ctx.stroke();
    ctx.restore();

    /* the altitude actually flown */
    ctx.strokeStyle = rgba(COL.accent, 1); ctx.lineWidth = 1.8;
    ctx.lineJoin = "round";
    ctx.beginPath(); first = true;
    for (k = 0; k < hLen; k++) {
      i = (hHead + k) % CAP;
      if (hT[i] < t0) continue;
      var x2 = px(hT[i]), y2 = py(hY[i]);
      if (first) { ctx.moveTo(x2, y2); first = false; } else ctx.lineTo(x2, y2);
    }
    ctx.stroke();
    ctx.restore();

    /* live dot at the right edge */
    var dx = px(t1), dy = py(y);
    if (dx > padL && dy > padT - 4 && dy < padT + ih + 4) {
      ctx.fillStyle = holding ? rgba(COL.run, 1) : rgba(COL.accent, 1);
      ctx.beginPath(); ctx.arc(Math.min(dx, w - padR - 1), dy, 2.8, 0, 6.2832); ctx.fill();
    }
  }

  function render() { drawScene(); drawPlot(); }

  /* ── readouts ───────────────────────────────────────────────────────────── */
  var lastUi = -1;
  function updateUi(force) {
    if (!force && simT - lastUi < 0.1) return;
    lastUi = simT;

    hAlt.textContent = y.toFixed(2);
    hVs.textContent = (v > -0.005 ? "+" : "−") + Math.abs(v).toFixed(2);
    hThr.textContent = String(Math.round(u / U_MAX * 100));

    var elapsed = ep ? simT - ep.t0 : 0;
    var real = ep && ep.dev > 0.08;

    if (!real) { sOver.textContent = "—"; sOver.className = "v"; }
    else {
      var op = Math.round(ep.over * 100);
      if (op > 999) op = 999;
      sOver.innerHTML = op + '<span class="u">%</span>';
      sOver.className = "v" + (op >= 40 ? " warn" : (op <= 12 && ep.settled ? " good" : ""));
    }

    if (ep && ep.settled) {
      sSettle.innerHTML = ep.settleT.toFixed(1) + '<span class="u">s</span>';
      sSettle.className = "v" + (ep.settleT <= 3 ? " good" : (ep.settleT > 8 ? " warn" : ""));
    } else if (elapsed > 20) {
      sSettle.innerHTML = '&gt;20<span class="u">s</span>';
      sSettle.className = "v bad";
    } else { sSettle.textContent = "—"; sSettle.className = "v"; }

    var cm = ssErr * 100;
    sErr.innerHTML = (cm < 100 ? "±" + cm.toFixed(cm < 10 ? 1 : 0) : "±" + (cm / 100).toFixed(1)) +
                     '<span class="u">' + (cm < 100 ? "cm" : "m") + '</span>';
    sErr.className = "v" + (ep && ep.settled && ssErr < 0.06 ? " good" : (ssErr > 0.5 ? " warn" : ""));

    var cls = "pill", txt;
    if (!userRunning) { txt = "paused"; }
    else if (y <= 0.01 && sp > 0.3) { cls = "pill bad"; txt = "on the ground"; }
    else if (ep && ep.settled && ssErr < 0.10) { cls = "pill on"; txt = "holding"; }
    else if (elapsed > 12 && !(ep && ep.settled)) { cls = "pill bad"; txt = "not settling"; }
    else { cls = "pill work"; txt = "correcting"; }
    pill.className = cls; pillText.textContent = txt;
  }

  /* ── loop ───────────────────────────────────────────────────────────────── */
  var userRunning = true, active = false, rafId = 0, lastT = 0;

  function frame(now) {
    rafId = requestAnimationFrame(frame);
    var dt = (now - lastT) / 1000;
    lastT = now;
    if (!(dt > 0)) return;
    if (dt > 0.1) dt = 0.1;
    advance(dt);
    render();
    updateUi(false);
  }
  function start() {
    if (active) return;
    active = true; lastT = performance.now(); acc = 0;
    rafId = requestAnimationFrame(frame);
  }
  function stop() {
    if (!active) return;
    active = false; cancelAnimationFrame(rafId);
  }
  function setStatus() {
    playBtn.textContent = userRunning ? "Pause" : "Play";
    playBtn.setAttribute("aria-label", userRunning ? "Pause the simulation" : "Play the simulation");
    overlay.hidden = userRunning;
    updateUi(true);
  }
  function setRunning(on) {
    userRunning = on;
    if (!on) { overlayNote.textContent = "Paused. Gust, test step and the presets each play a couple of seconds so you can still read the answer."; dismissHint(); }
    if (on && !document.hidden) start(); else stop();
    setStatus();
    say(on ? "Flying." : "Paused.");
  }

  /* ── actions ────────────────────────────────────────────────────────────── */

  /* While paused (reduced motion parks the demo on purpose) every action still
     has to answer, so it plays a couple of seconds of flight instead of freezing
     on a line that moved and a quad that did not. */
  var PAUSE_PEEK = 2.5;
  function peek() {
    if (active) return;
    advance(PAUSE_PEEK);
    render(); updateUi(true);
  }

  /* The standard exam. Three tunes are only worth comparing if they sit the same
     paper, so this trims the quad to a dead-steady hover at TEST_LO (Ki*I cancels
     gravity exactly), back-fills a flat baseline on the plot, holds a beat, then
     asks for TEST_HI. Same start, same 3 m step, same direction, every time —
     which is what makes "sluggish takes 18 s, well tuned takes 2.5 s" a fact
     rather than an accident of which chip you clicked first. */
  var TEST_LO = 3.0, TEST_HI = 6.0;

  function armTestStep() {
    y = TEST_LO; v = 0; u = G; sway = 0; swayV = 0;
    I = Ki > 0.01 ? G / Ki : 0;
    sp = TEST_LO;
    clearHistory();
    for (var b = 7.0; b > 1e-6; b -= SAMPLE) pushSample(simT - b, TEST_LO, TEST_LO);
    newEpisode("trim");
    var n = Math.round(1.0 / SUB);
    for (var i = 0; i < n; i++) substep(SUB);
    sp = TEST_HI;
    newEpisode("step");
  }
  function flyTestStep() {
    armTestStep();
    dismissHint();
    peek();
  }
  function testStepSaid() {
    return "Flying the standard " + TEST_LO.toFixed(0) + " to " + TEST_HI.toFixed(0) + " metre test step.";
  }

  function setSetpoint(alt, how) {
    alt = Math.max(0.4, Math.min(10.4, Math.round(alt * 20) / 20));
    if (Math.abs(alt - sp) < 0.001) return;
    sp = alt;
    newEpisode("step");
    dismissHint();
    peek();
    say("Now holding " + sp.toFixed(2) + " metres" + (how ? " (" + how + ")" : "") + ".");
  }

  function doGust() {
    var down = Math.random() < 0.72;
    gustAmp = (down ? -1 : 1) * (7 + Math.random() * 4);
    gustDur = 0.34 + Math.random() * 0.16;
    gustT0 = simT;
    gusts.push(simT);
    while (gusts.length && gusts[0] < simT - WINDOW - 2) gusts.shift();
    newEpisode("gust");
    dismissHint();
    peek();
    updateUi(true);
    say(down ? "Downdraft. Watch it recover." : "Updraft. Watch it recover.");
  }

  function takeoff() {
    y = 0; v = 0; I = 0; u = 0; sway = 0; swayV = 0;
    simT += 0.001;
    clearHistory();
    newEpisode("takeoff");
    pushSample(simT, y, sp);
    if (!active) { render(); }
    updateUi(true);
    say("Landed. Taking off again to " + sp.toFixed(2) + " metres.");
  }

  /* ── gains + presets ────────────────────────────────────────────────────── */
  var PRESETS = [
    { name: "sluggish",   kp: 4,  ki: 0.5,  kd: 5.0, note: "Sluggish: barely any push, so it drifts up to the line like it is thinking about something else — 11% over, and 18 s before it stops creeping." },
    { name: "twitchy",    kp: 28, ki: 12,   kd: 0.8, note: "Twitchy: plenty of push, almost no brakes. It sails 95% past the line and rings for another ten seconds." },
    { name: "well tuned", kp: 26, ki: 5,    kd: 7.5, note: "Well tuned: nearly the same push as twitchy, but real brakes behind it. 5% overshoot, on the line in 2.5 s." }
  ];

  function syncSlider(key) {
    var val = parseFloat(sliders[key].value);
    if (key === "kp") { Kp = val; vals.kp.textContent = val.toFixed(1); }
    else if (key === "ki") { Ki = val; vals.ki.textContent = val.toFixed(2); }
    else { Kd = val; vals.kd.textContent = val.toFixed(2); }
  }
  function markPreset() {
    var btns = presetsEl.querySelectorAll("button"), hit = -1;
    for (var i = 0; i < PRESETS.length; i++) {
      var p = PRESETS[i];
      if (Math.abs(p.kp - Kp) < 1e-6 && Math.abs(p.ki - Ki) < 1e-6 && Math.abs(p.kd - Kd) < 1e-6) hit = i;
    }
    for (var b = 0; b < btns.length; b++) btns[b].setAttribute("aria-pressed", String(b === hit));
    return hit;
  }

  Object.keys(sliders).forEach(function (key) {
    sliders[key].addEventListener("input", function () {
      syncSlider(key);
      var hit = markPreset();
      if (hit < 0) presetNote.textContent = "Your own tune. Hit test step to sit it the same 3 → 6 m exam as the presets, or gust to see it take a punch.";
      else presetNote.textContent = PRESETS[hit].note;
      if (!active) { render(); updateUi(true); }
    });
  });

  presetsEl.addEventListener("click", function (e) {
    var btn = e.target.closest ? e.target.closest("button[data-p]") : null;
    if (!btn) return;
    var p = PRESETS[parseInt(btn.getAttribute("data-p"), 10)];
    sliders.kp.value = p.kp; sliders.ki.value = p.ki; sliders.kd.value = p.kd;
    syncSlider("kp"); syncSlider("ki"); syncSlider("kd");
    markPreset();
    presetNote.textContent = p.note;
    /* always re-fly the standard step — otherwise a second preset click looks
       dead, and the metrics still read from whatever the previous tune did */
    flyTestStep();
    say(p.name + " tune loaded. Kp " + p.kp + ", Ki " + p.ki + ", Kd " + p.kd + ". " + testStepSaid());
  });

  gustBtn.addEventListener("click", doGust);
  takeoffBtn.addEventListener("click", takeoff);
  playBtn.addEventListener("click", function () { setRunning(!userRunning); });
  overlayPlay.addEventListener("click", function () { setRunning(true); scene.focus(); });
  upBtn.addEventListener("click", function () { flyTestStep(); say(testStepSaid()); });

  /* ── pointer + keyboard on the scene ────────────────────────────────────── */
  function altFromEvent(e) {
    var r = scene.getBoundingClientRect();
    var h = r.height;
    var padT = 16, padB = 30;
    var span = h - padT - padB;
    var py2 = e.clientY - r.top;
    return ((h - padB) - py2) / span * ALT_VIEW;
  }
  scene.addEventListener("pointerdown", function (e) {
    if (e.button !== undefined && e.button !== 0) return;
    setSetpoint(altFromEvent(e), "clicked");
    scene.focus();
  });
  scene.addEventListener("keydown", function (e) {
    var k = e.key, stepq = e.shiftKey ? 1 : 0.25;
    if (k === "ArrowUp") { e.preventDefault(); setSetpoint(sp + stepq, "keyboard"); }
    else if (k === "ArrowDown") { e.preventDefault(); setSetpoint(sp - stepq, "keyboard"); }
    else if (k === "Enter" || k === " " || k === "Spacebar") { e.preventDefault(); doGust(); }
  });

  /* ── announcements + hint ───────────────────────────────────────────────── */
  var sayTimer = 0;
  function say(msg) {
    clearTimeout(sayTimer);
    sayTimer = setTimeout(function () { live.textContent = msg; }, 140);
  }
  var hintTimer = 0;
  function dismissHint() {
    if (hint.hidden || hint.classList.contains("gone")) return;
    hint.classList.add("gone");
    clearTimeout(hintTimer);
    setTimeout(function () { hint.hidden = true; }, 500);
  }

  /* ── lifecycle ──────────────────────────────────────────────────────────── */
  document.addEventListener("visibilitychange", function () {
    if (document.hidden) stop();
    else if (userRunning) start();
  });
  if (window.ResizeObserver) new ResizeObserver(function () { resize(); }).observe(field);
  window.addEventListener("resize", resize);

  /* ── boot: fly the first twelve seconds before anyone looks ──────────────── */
  readTheme();
  syncSlider("kp"); syncSlider("ki"); syncSlider("kd");
  markPreset();

  /* Fly the standard step before anyone looks, so the very first paint is a
     finished measurement — flat hover, the ask, the bounce, the recovery — and
     clicking "test step" reproduces it exactly. */
  simT = 20;                      /* keeps the back-filled baseline off t < 0 */
  armTestStep();
  var wi, w1 = Math.round(6.5 / SUB);
  for (wi = 0; wi < w1; wi++) substep(SUB);

  resize();
  updateUi(true);

  if (reduced) {
    userRunning = false;
    overlayNote.textContent = "Reduced motion is on, so the quad is parked mid-hover. Gust, test step and the presets each play a couple of seconds, so every control still answers.";
    setStatus();
    hint.hidden = true;
  } else {
    setStatus();
    start();
    hintTimer = setTimeout(dismissHint, 8000);
  }
})();
</script>
</body>
</html>