refactor(studio): give resolveTimelineMove a row-based vertical axis

Rows stopped sharing one pixel height when lanes gained expansion, so the only
production caller was passing cumulative row coordinates with trackHeight 1 and
both scrollTops zeroed. The parameter names described units the values no longer
carried. The vertical axis is now a row index and the caller keeps ownership of
folding scroll and per-row heights into it.

packages/studio/src/player/components/timelineClipDragPreview.ts

@@ -139,22 +139,18 @@ export function computeDragPreview(
     ctx.rowHeights,
   );
   const currentRow = getTimelineRowFromY(clientY - scrollRectTop + scrollTop, ctx.rowHeights);
-  // resolveTimelineMove's vertical axis is expressed in track-height units.
-  // Feeding cumulative row coordinates with a unit height preserves its existing
-  // threshold/create-track behavior while supporting variable pixel heights.
+  // resolveTimelineMove's vertical axis is row indices, which is why the pointer
+  // and scroll pixels are folded into originRow/currentRow above.
   const nextMove = resolveTimelineMove(
     {
       start: drag.element.start,
       track: drag.element.track,
       duration: drag.element.duration,
       originClientX: drag.originClientX,
-      originClientY: originRow,
+      originRow,
       originScrollLeft: drag.originScrollLeft,
-      originScrollTop: 0,
       currentScrollLeft: scroll?.scrollLeft ?? drag.originScrollLeft,
-      currentScrollTop: 0,
       pixelsPerSecond: pps,
-      trackHeight: 1,
       maxStart: dragMaxStart,
       trackOrder,
     },

packages/studio/src/player/components/timelineEditing.test.ts

@@ -27,14 +27,13 @@ describe("resolveTimelineMove", () => {
           track: 2,
           duration: 2,
           originClientX: 100,
-          originClientY: 200,
+          originRow: 0,
           pixelsPerSecond: 100,
-          trackHeight: 72,
           maxStart: 8,
           trackOrder: [0, 1, 2, 3, 4],
         },
         245,
-        200,
+        0,
       ),
     ).toEqual({ start: 2.7, track: 2 });
   });
@@ -47,14 +46,13 @@ describe("resolveTimelineMove", () => {
           track: 1,
           duration: 3,
           originClientX: 200,
-          originClientY: 200,
+          originRow: 0,
           pixelsPerSecond: 100,
-          trackHeight: 72,
           maxStart: 10,
           trackOrder: [0, 1, 5, 9],
         },
         150,
-        390,
+        190 / 72,
       ),
     ).toEqual({ start: 1.5, track: 9 });
   });
@@ -67,14 +65,13 @@ describe("resolveTimelineMove", () => {
           track: 0,
           duration: 4,
           originClientX: 300,
-          originClientY: 200,
+          originRow: 0,
           pixelsPerSecond: 100,
-          trackHeight: 72,
           maxStart: 6,
           trackOrder: [0, 10, 20],
         },
         -100,
-        -200,
+        -400 / 72,
       ),
     ).toEqual({ start: 0, track: -1 });
 
@@ -85,14 +82,13 @@ describe("resolveTimelineMove", () => {
           track: 10,
           duration: 4,
           originClientX: 300,
-          originClientY: 200,
+          originRow: 0,
           pixelsPerSecond: 100,
-          trackHeight: 72,
           maxStart: 6,
           trackOrder: [0, 10, 20],
         },
         500,
-        200,
+        0,
       ),
     ).toEqual({ start: 6, track: 10 });
   });
@@ -105,14 +101,13 @@ describe("resolveTimelineMove", () => {
           track: 0,
           duration: 2,
           originClientX: 100,
-          originClientY: 200,
+          originRow: 0,
           pixelsPerSecond: 100,
-          trackHeight: 72,
           maxStart: 8,
           trackOrder: [0, 10, 20],
         },
         100,
-        150,
+        -50 / 72,
       ),
     ).toEqual({ start: 1, track: -1 });
   });
@@ -125,19 +120,18 @@ describe("resolveTimelineMove", () => {
           track: 20,
           duration: 2,
           originClientX: 100,
-          originClientY: 200,
+          originRow: 0,
           pixelsPerSecond: 100,
-          trackHeight: 72,
           maxStart: 8,
           trackOrder: [0, 10, 20],
         },
         100,
-        250,
+        50 / 72,
       ),
     ).toEqual({ start: 1, track: 21 });
   });
 
-  it("accounts for scroll displacement while dragging", () => {
+  it("accounts for horizontal scroll displacement while dragging", () => {
     expect(
       resolveTimelineMove(
         {
@@ -145,18 +139,17 @@ describe("resolveTimelineMove", () => {
           track: 0,
           duration: 2,
           originClientX: 100,
-          originClientY: 200,
+          originRow: 0,
           originScrollLeft: 0,
-          originScrollTop: 0,
           currentScrollLeft: 100,
-          currentScrollTop: 144,
           pixelsPerSecond: 100,
-          trackHeight: 72,
           maxStart: 8,
+          // Vertical scroll never reaches here: the drag preview folds it into the
+          // row index it passes, because rows no longer share one pixel height.
           trackOrder: [0, 1, 2, 3],
         },
         100,
-        200,
+        2,
       ),
     ).toEqual({ start: 2, track: 2 });
   });
@@ -195,9 +188,8 @@ describe("resolveTimelineMove", () => {
         track: 1,
         duration: 2,
         originClientX: 0,
-        originClientY: 0,
+        originRow: 0,
         pixelsPerSecond: 100,
-        trackHeight: 72,
         maxStart: 8,
         trackOrder: [0, 1],
         layerOrder: layers.map((layer) => layer.id),
@@ -206,7 +198,7 @@ describe("resolveTimelineMove", () => {
         stackingElements,
       },
       0,
-      -72,
+      -1,
     );
 
     expect(result).toEqual({

packages/studio/src/player/components/timelineEditing.ts

@@ -46,13 +46,11 @@ export interface TimelineMoveInput {
   track: number;
   duration: number;
   originClientX: number;
-  originClientY: number;
+  /** Vertical position as a track-row index, not pixels: rows vary in height. */
+  originRow: number;
   originScrollLeft?: number;
-  originScrollTop?: number;
   currentScrollLeft?: number;
-  currentScrollTop?: number;
   pixelsPerSecond: number;
-  trackHeight: number;
   maxStart: number;
   trackOrder: number[];
   layerOrder?: TimelineLayerId[];
@@ -108,7 +106,7 @@ export function resolveTimelineAutoScroll(
 export function resolveTimelineMove(
   input: TimelineMoveInput,
   clientX: number,
-  clientY: number,
+  currentRow: number,
 ): {
   start: number;
   track: number;
@@ -117,11 +115,12 @@ export function resolveTimelineMove(
   stackingReorder?: TimelineStackingReorderIntent | null;
 } {
   const scrollDeltaX = (input.currentScrollLeft ?? 0) - (input.originScrollLeft ?? 0);
-  const scrollDeltaY = (input.currentScrollTop ?? 0) - (input.originScrollTop ?? 0);
   const deltaTime =
     (clientX - input.originClientX + scrollDeltaX) / Math.max(input.pixelsPerSecond, 1);
-  const trackDeltaRaw =
-    (clientY - input.originClientY + scrollDeltaY) / Math.max(input.trackHeight, 1);
+  // Rows, so vertical scroll and per-row heights are the caller's problem: rows
+  // have varied in height since lanes expand, and a single trackHeight can't
+  // describe them.
+  const trackDeltaRaw = currentRow - input.originRow;
   const deltaTrack = Math.round(trackDeltaRaw);
   const nextStart = clamp(
     roundToCentiseconds(input.start + deltaTime),