instrument: a deck group's reserved cell width goes to the cells present

A Trigger face dropping Sustain and Release now gets wider cells instead of
144 px of dead slots. Group widths, row packing and Gate are untouched.

src/core/instrument/CLAUDE.md

@@ -307,7 +307,7 @@ anything for a trigger shape.
 - `browser_scroll` — scroll + type-to-filter layered over `capture_browser`: vertical scroll offset, scrollbar thumb, thumb-drag mapping, and name-substring search.
 - `param_slider` — parameter control-panel: vertical stack of TOGGLE (two-segment selector) and SLIDER (horizontal track) rows; maps normalized value to/from handle pixel.
 - `embed_strip` — compact single-row control layout for embed mode in the track FX chain.
-- `knob_deck` — pure knob-deck layout + hit-test (FB1): group-box / caption-row / compact-toggle / knob-cell geometry, deterministic whole-group wrap, `DeckLayout` / `DeckHit`. Mirror of `action_bar`/`param_slider`; no LICE or REAPER types. A group carries TWO caption-toggle slots, laid right-to-left: the second exists because a group whose knob row is wider than its caption row has caption slack a toggle can occupy for free, and the deck has six pixels of headroom on its first row at the editor's floor width — a `rowToggle` would widen the GROUP and wrap the deck to a fourth row, past what the minimum window holds.
+- `knob_deck` — pure knob-deck layout + hit-test (FB1): group-box / caption-row / compact-toggle / knob-cell geometry, deterministic whole-group wrap, `DeckLayout` / `DeckHit`. Mirror of `action_bar`/`param_slider`; no LICE or REAPER types. A group carries TWO caption-toggle slots, laid right-to-left: the second exists because a group whose knob row is wider than its caption row has caption slack a toggle can occupy for free, and the deck has six pixels of headroom on its first row at the editor's floor width — a `rowToggle` would widen the GROUP and wrap the deck to a fourth row, past what the minimum window holds. **A group's cell run is a RESERVED WIDTH, not a fixed cell size**: a `-1` id reserves one cell's width without a cell, and the cells present divide the whole run between them at one uniform integer width (residue in symmetric end margins). That is what lets a mode flip drop controls from a face — Trigger's AMP and FILTER ENV lose their Sustain/Release stages — without either reflowing the deck or leaving dead slots in the box; a face with fewer controls simply gets roomier cells. Do not reintroduce fixed-width cells with blank slots.
 - `deck_groups` — also home to `isLiveDeckParam` and `liveCommitFor`, the editor's whole commit-tier routing decision (see "Live parameter delivery" above), and to `OverlayEnv` + `nextOverlaySelection`/`overlayEnvEnabled`/`overlayEnvInert`, the whole overlay-selection state machine (exclusivity, the none resting state, and which selections a disabled or DRAWN group makes inert); WHICH groups the Sample face's deck carries, split from `knob_deck`'s HOW they lay out: the `DeckParam` control-id space (the editor's `ParamControl` is an alias of it), the `DeckGroupId` list, `sampleDeckGroups` in signal-flow order (**pitch → filter → amp**, then velocity/voice/master), and the deck's bipolar-knob law. Reads `PlayMode` for the AMP group's Gate/Trigger face, which is why this and not `knob_deck` is the module that touches the engine's value layer. Also home to `CurveTarget` + `curveTargetFor` — the VELOCITY group's three cells are popup openers, not dials, and that predicate is the ONE place they are named, so paint, hit-test routing and the popup's title all agree. MASTER is reserved for post-voice-mixer concerns, which is why the curves sit in their own group immediately left of VOICE rather than there.
 - `spline_edit` — THE point-editing grammar, and the one place it is written down: left-click grabs a node and adds one in empty space, right-click deletes, control-click toggles hard/smooth. Both spline consumers — the velocity-curve popup and the spline EG overlay — route their mouse-down through `resolveSplineEdit`, so the two cannot drift into two grammars. The endpoint and point-count rules are NOT restated here: `deletePoint` and `addPoint` own them, and the caller applies the resolved action to the curve. Also home to `splineOverlayBox`, the contour's mapping box inside the waveform overlay — the FULL area, no inset, so the drawn contour stays 1:1 with the sample's time axis.
 - `curve_popup` — pure curve-popup geometry + dismissal test (FB1): centered sheet over the Sample face — width/height clamps, title row, Close button rect, curve-box rect, outside-sheet dismissal test. Mirror of `overflow_menu`; no LICE or REAPER types.

src/core/instrument/ui/deck_groups.cpp

@@ -88,8 +88,9 @@ std::vector<DeckGroupDesc> sampleDeckGroups(PlayMode playMode) {
         amp.captionToggle2 = {id(DeckParam::kAmpEnvMode), kEnvModeSegW};
         if (trigger) {
             // The play span first, then the AHD that shapes it, time-ordered left-to-right so
-            // the row reads like the drawn envelope. One blank keeps the group's width — and
-            // therefore its neighbours' placement — identical across a mode flip.
+            // the row reads like the drawn envelope. One reserve (-1) keeps the group's width —
+            // and therefore its neighbours' placement — identical across a mode flip; its
+            // pixels go to the four cells that remain (knob_deck.h).
             amp.cellIds = {id(DeckParam::kTrigLength), id(DeckParam::kTrigAttack),
                            id(DeckParam::kTrigHold), id(DeckParam::kTrigDecay), -1};
         } else {

src/core/instrument/ui/deck_groups.h

@@ -111,8 +111,8 @@ CurveTarget curveTargetFor(int controlId);
 
 // The deck's groups, left to right, in SIGNAL-FLOW order: pitch -> filter -> amp, then the
 // two instance-wide groups. `playMode` picks the AMP and FILTER ENV groups' faces — AHDSR in
-// Gate, AHD in Trigger — via knob_deck's blank-cell reservation (knob_deck.h) so a mode flip
-// never reflows the neighbouring groups.
+// Gate, AHD in Trigger — via knob_deck's cell-width reservation (knob_deck.h) so a mode flip
+// never reflows the neighbouring groups and never leaves a hole in the narrower face.
 std::vector<DeckGroupDesc> sampleDeckGroups(PlayMode playMode);
 
 // The curve-exponent control a stage knob's INNER DIAL edits, or kCount when the knob shapes

src/core/instrument/ui/knob_deck.cpp

@@ -65,14 +65,23 @@ DeckGroupLayout layoutGroup(const DeckGroupDesc& g, const Rect& box) {
     placeToggle(g.captionToggle, out.captionToggle);
     placeToggle(g.captionToggle2, out.captionToggle2);
 
-    // Knob row: fixed cells left-to-right, then the optional row toggle.
+    // Knob row: the cells present divide the whole reserved run (one kDeckCellW per declared
+    // id, reserves included). Integer division puts an indivisible residue in symmetric end
+    // margins rather than in one odd-width cell — keyboard_strip's uniformity-wins rule.
     const int cellTop = captionTop + kDeckCaptionH + kDeckCaptionGap;
-    int x = innerLeft;
+    const int runWidth = static_cast<int>(g.cellIds.size()) * kDeckCellW;
+    int presentCells = 0;
     for (int id : g.cellIds) {
+        if (id >= 0) ++presentCells;
+    }
+    const int cellW = presentCells > 0 ? runWidth / presentCells : 0;
+    int x = innerLeft + (runWidth - presentCells * cellW) / 2;
+    for (int id : g.cellIds) {
+        if (id < 0) continue;
         DeckCellLayout c;
         c.id = id;
-        c.cell = Rect::ltrb(x, cellTop, x + kDeckCellW, cellTop + kDeckCellH);
-        const int knobLeft = x + (kDeckCellW - kDeckKnobSize) / 2;
+        c.cell = Rect::ltrb(x, cellTop, x + cellW, cellTop + kDeckCellH);
+        const int knobLeft = x + (cellW - kDeckKnobSize) / 2;
         const int knobTop = cellTop + 4;
         c.knob = Rect::ltrb(knobLeft, knobTop, knobLeft + kDeckKnobSize, knobTop + kDeckKnobSize);
         const int innerLeftPx = knobLeft + (kDeckKnobSize - kDeckInnerDialSize) / 2;
@@ -82,13 +91,16 @@ DeckGroupLayout layoutGroup(const DeckGroupDesc& g, const Rect& box) {
         const int labelTop = knobTop + kDeckKnobSize + 4;
         c.label = Rect::ltrb(c.cell.x, labelTop, c.cell.right(), labelTop + kDeckCellLabelH);
         out.cells.push_back(c);
-        x += kDeckCellW;
+        x += cellW;
     }
     if (g.rowToggle.id >= 0) {
-        if (!g.cellIds.empty()) x += kDeckToggleGap;
+        // Anchored past the whole reserved run, not past the last cell, so a residue margin
+        // cannot shift it.
+        int tx = innerLeft + runWidth;
+        if (!g.cellIds.empty()) tx += kDeckToggleGap;
         const int segW = g.rowToggle.segWidth;
         const int togTop = cellTop + (kDeckCellH - kDeckToggleH) / 2;
-        const Rect seg0 = Rect::ltrb(x, togTop, x + segW, togTop + kDeckToggleH);
+        const Rect seg0 = Rect::ltrb(tx, togTop, tx + segW, togTop + kDeckToggleH);
         const Rect seg1 = Rect::ltrb(seg0.right(), togTop, seg0.right() + segW, togTop + kDeckToggleH);
         out.rowToggle = DeckToggleLayout{g.rowToggle.id, seg0, seg1};
     }

src/core/instrument/ui/knob_deck.h

@@ -5,9 +5,9 @@
 //
 // The deck is a horizontal run of fenced groups, left->right, each a bordered box with a
 // caption row (caption left, the group's compact mode toggle right-anchored) over a knob
-// row of fixed cells (knob centered, label band beneath). A group may also place one
+// row of equal-width cells (knob centered, label band beneath). A group may also place one
 // two-segment toggle in the knob row after its cells. Groups that must keep stable
-// geometry across a mode flip reserve blank cells (id -1) so a mode flip never reflows
+// geometry across a mode flip reserve cell width (id -1) so a mode flip never reflows
 // neighbouring groups.
 //
 // Wrap is deterministic: groups place left-to-right with kDeckGroupGap between; a group
@@ -57,7 +57,8 @@ struct DeckRadioDesc {
 };
 
 // One fenced group, in deck order. `cellIds` are the knob cells left-to-right; an id of -1
-// is a reserved blank cell (geometry held, never hit). `captionWidth` is the px the shell
+// reserves one cell's WIDTH without a cell, and the cells present divide the whole run —
+// see this module's CLAUDE.md bullet for what that buys. `captionWidth` is the px the shell
 // reserves for the caption text (this module does not measure text).
 struct DeckGroupDesc {
     int id = 0;                    // shell group id (opaque here)
@@ -87,7 +88,7 @@ struct DeckRadioLayout {
 
 struct DeckCellLayout {
     int id = -1;
-    Rect cell;   // the full 48x58 cell
+    Rect cell;   // the whole cell; width is the group's reserved run divided by its cell count
     Rect knob;   // the centered kDeckKnobSize square (the knob circle inscribes it)
     Rect inner;  // the concentric kDeckInnerDialSize square inside `knob`
     Rect label;  // the 12px label band beneath the knob
@@ -141,7 +142,7 @@ struct DeckHit {
 // The deck element a point lands on: a knob cell (the whole cell, not just the knob
 // circle — the shell anchors the vertical drag wherever the grab lands, with `inner` marking
 // a grab on the concentric inner dial), a caption-toggle segment, a row-toggle segment, or
-// the caption-row corner radio. Blank cells (id -1) and everything else miss.
+// the caption-row corner radio. Everything else — fence, padding, outside — misses.
 DeckHit hitTestDeck(const DeckLayout& layout, int x, int y);
 
 }  // namespace reasampler::instrument::ui