Let a texture frame set where a sprite stands for Y sorting (#228)

A sprite sorted by Y stood on the bottom of its box. With a shadow or
empty rows under the feet that is too low, so a figure slipped in
front of a tree it should stand behind. The line belongs to the frame,
not the texture, because one texture can hold several sprite sheets.

- New field `sortLine` on a frame: whole image pixels up from the
  frame's bottom edge, 0 to the frame height. 0 is stored as nothing,
  so old projects stay the same. Shrinking a frame keeps the line
  inside it.
- `feetOf` in drawOrder.ts lifts a sprite by the same share of its
  box. Scene view, game view, play and picking all use it. In an
  animation the line of the frame shown right now counts.
- The texture tab draws the selected frame's line dashed in yellow
  with a knob left of the frame. At 0 the line shares the bottom edge
  with the nine-slice border and a resize handle, so the knob is
  tested first. A line inside the frame can be dragged directly,
  tested after the nine-slice lines.
- The inspector has a new "Sorting" section with a "Sort line"
  field. Dragging or typing is one undo step each.

Limit: frames of one walk cycle need the same value, set by hand.
Otherwise the figure changes its place in the order while walking.

Tests:
- ./check.sh: 160 files, 2356 tests, CHECK EXIT 0 (before 2350).
- 6 new unit tests in tests/viewport/sort-by-y.test.ts.
- e2e 37 and 39-43: ALL PASSED, 162 checks. New check 37b-ii drags
  the knob up 6 px and undoes it, plus the inspector row.
- Counter checks: if feetOf ignores the line, exactly the 3 order
  tests fail. If the knob cannot be grabbed, 37b-ii fails.
- The e2e sum in the README is 1380, counted, not yet seen in CI.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/editor/texture/TextureCanvas.ts

@@ -4,6 +4,7 @@ import {
   SetFrameCollisionCommand,
   SetFrameRectCommand,
   SetFrameSliceCommand,
+  SetFrameSortLineCommand,
 } from '../../core/commands/frameCommands';
 import { polygonProblem } from '../../core/model/polygon';
 import type { FrameBorders, TextureAsset, TextureFrame } from '../../core/model/Asset';
@@ -14,12 +15,15 @@ import {
   frameCollision,
   frameFill,
   type FrameRect,
+  frameSortLine,
   MIN_FRAME_SIZE,
   normalizeFrameRect,
+  normalizeSortLine,
   rawFrameSlice,
   restoreFrameSlice,
   setFrameCollision,
   setFrameSlice,
+  setFrameSortLine,
 } from '../../core/model/texture';
 import { pressCount, Theme, textStyle, type Widget } from '../../ui';
 import {
@@ -55,6 +59,8 @@ import {
   localToImage,
   rectBetween,
   type SheetView,
+  sortLineAt,
+  sortLineY,
 } from './sheetView';
 
 /** Side of a resize handle in screen pixels – constant, so handles stay grabbable at any zoom. */
@@ -66,12 +72,18 @@ const MIN_LABEL_SIZE = 28;
 /** Inset of a frame's name plate from the frame's top-left corner, and its own padding. */
 const LABEL_INSET = 3;
 const LABEL_PAD = 2;
+/** The sort line's knob, left of the frame: its gap to the frame and its size, in screen pixels. */
+const SORT_KNOB_GAP = 4;
+const SORT_KNOB_SIZE = 8;
 
-/** Either a handle of the frame's rectangle or one of its nine-slice border lines – never both. */
+/** A handle of the frame's rectangle, one of its nine-slice border lines, or its sort line. */
 interface FrameDrag {
   frameId: string;
   handle: FrameHandle | null;
   edge: BorderEdge | null;
+  sortLine: boolean;
+  /** The line as it was, raw, so the command records the state that was really there. */
+  startSortLine: number | undefined;
   start: FrameRect;
   startBorder: FrameBorders;
   /** The slice keys as they were, so the command records the state that was really there. */
@@ -185,12 +197,25 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
       sheetOutlines: this.sheetOutlines,
       filled: { sheet: this.sheetFilled, overlay: this.overlayFilled },
       origin: { x: origin.x, y: origin.y },
+      sortKnob: this.sortKnob(origin),
       labels: [...this.labels.values()]
         .filter((label) => label.visible)
         .map((label) => ({ text: label.text, ...label.getGlobalPosition() })),
     };
   }
 
+  /** The middle of the selected frame's sort-line knob on screen, or null without one to grab. */
+  private sortKnob(origin: Vec2): Vec2 | null {
+    const frame = this.frame(this.selectedFrameId());
+    if (!frame || this.collision) return null;
+    const at = imageToLocal(
+      { panX: 0, panY: 0, zoom: this.view.zoom },
+      frame.x,
+      sortLineY(frame, frameSortLine(frame)),
+    );
+    return { x: origin.x + at.x - SORT_KNOB_GAP - SORT_KNOB_SIZE / 2, y: origin.y + at.y };
+  }
+
   private asset(): TextureAsset | null {
     const asset = this.assetId ? findAsset(this.state.project, this.assetId) : undefined;
     return asset && asset.type === 'texture' ? asset : null;
@@ -256,6 +281,12 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
     const selected = this.frame(this.selectedFrameId());
     const onSelected = selected ? handleAt(selected, point, tolerance) : null;
     const onBorder = selected ? borderAt(selected, frameBorders(selected), point, tolerance) : null;
+    const onSortLine = selected ? this.sortLineAt(selected, point, tolerance) : null;
+    if (selected && onSortLine === 'knob') {
+      this.startDrag(selected, null, null, local, true);
+      e.stopPropagation();
+      return;
+    }
     if (selected && onSelected && onSelected !== 'move') {
       this.startDrag(selected, onSelected, null, local);
       e.stopPropagation();
@@ -266,6 +297,11 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
       e.stopPropagation();
       return;
     }
+    if (selected && onSortLine) {
+      this.startDrag(selected, null, null, local, true);
+      e.stopPropagation();
+      return;
+    }
     if (selected && onSelected) {
       this.startDrag(selected, onSelected, null, local);
       e.stopPropagation();
@@ -386,11 +422,14 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
     handle: FrameHandle | null,
     edge: BorderEdge | null,
     local: Vec2,
+    sortLine = false,
   ): void {
     this.dragging = {
       frameId: frame.id,
       handle,
       edge,
+      sortLine,
+      startSortLine: frame.sortLine,
       start: { x: frame.x, y: frame.y, w: frame.w, h: frame.h },
       startBorder: frameBorders(frame),
       startSlice: rawFrameSlice(frame),
@@ -448,7 +487,8 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
     const asset = this.asset();
     const frame = drag && this.frame(drag.frameId);
     if (!drag || !asset || !frame) return;
-    if (drag.edge)
+    if (drag.sortLine) setFrameSortLine(frame, normalizeSortLine(drag.startSortLine ?? 0, drag.start.h) - dy);
+    else if (drag.edge)
       setFrameSlice(frame, {
         border: dragBorder(drag.startBorder, drag.edge, dx, dy),
         fill: frameFill(frame),
@@ -483,7 +523,12 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
       const frame = this.frame(drag.frameId);
       if (drag.moved && asset && frame) {
         // The preview moved it already; put it back so the command records the real start.
-        if (drag.edge) {
+        if (drag.sortLine) {
+          const next = frameSortLine(frame);
+          if (drag.startSortLine === undefined) delete frame.sortLine;
+          else frame.sortLine = drag.startSortLine;
+          this.state.execute(new SetFrameSortLineCommand(asset.id, drag.frameId, next));
+        } else if (drag.edge) {
           const next = { border: frameBorders(frame), fill: frameFill(frame) };
           restoreFrameSlice(frame, drag.startSlice);
           this.state.execute(new SetFrameSliceCommand(asset.id, drag.frameId, next));
@@ -526,6 +571,11 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
     const tolerance = HANDLE_SIZE / this.view.zoom;
     const selected = this.frame(this.selectedFrameId());
     const handle = selected ? handleAt(selected, point, tolerance) : null;
+    const onSortLine = selected ? this.sortLineAt(selected, point, tolerance) : null;
+    if (onSortLine === 'knob') {
+      this.cursor = 'ns-resize';
+      return;
+    }
     if (handle && handle !== 'move') {
       this.cursor = handleCursor(handle);
       return;
@@ -536,6 +586,10 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
       this.cursor = borderCursor(edge);
       return;
     }
+    if (onSortLine) {
+      this.cursor = 'ns-resize';
+      return;
+    }
     if (handle) {
       this.cursor = handleCursor(handle);
       return;
@@ -622,6 +676,7 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
       this.overlayFilled = this.drawOutlines(this.overlay, view, selected, this.collision, true);
       if (!this.collision) {
         this.drawBorders(view, selected);
+        this.drawSortLine(view, selected);
         this.drawHandles(view, selected);
       }
     }
@@ -659,6 +714,26 @@ export class TextureCanvas extends SheetCanvas<TextureAsset> {
     this.overlay.stroke({ width: 1, color: Theme.colors.marquee });
   }
 
+  /** The sort line (#228) across the selected frame, with its knob left of it: a line at 0 is on the edge. */
+  private drawSortLine(view: SheetView, frame: TextureFrame): void {
+    const y = sortLineY(frame, frameSortLine(frame));
+    const a = imageToLocal(view, frame.x, y);
+    const b = imageToLocal(view, frame.x + frame.w, y);
+    dashedLine(this.overlay, a.x, a.y, b.x, b.y, 6, 3);
+    this.overlay.stroke({ width: 1, color: Theme.colors.hot });
+    const tip = a.x - SORT_KNOB_GAP;
+    const half = SORT_KNOB_SIZE / 2;
+    this.overlay
+      .poly([tip, a.y, tip - SORT_KNOB_SIZE, a.y - half, tip - SORT_KNOB_SIZE, a.y + half], true)
+      .fill({ color: Theme.colors.hot })
+      .stroke({ width: 1, color: Theme.colors.symbolOutline });
+  }
+
+  private sortLineAt(frame: TextureFrame, point: Vec2, tolerance: number): 'knob' | 'line' | null {
+    const reach = (SORT_KNOB_GAP + SORT_KNOB_SIZE + 2) / this.view.zoom;
+    return sortLineAt(frame, frameSortLine(frame), point, tolerance, reach);
+  }
+
   /**
    * A frame's collision outlines (#141), always shown so one sees which frames block with them; on
    * the edited frame the active outline is drawn heavier and every corner of every outline gets a