Prevent mobile trim release rollback

src/lib/timelineEdgeAutoScroll.js

@@ -51,6 +51,7 @@ export function createTimelineEdgeAutoScroller({ trackElement, pointerType, time
     (pointerType === "touch" && isMobile)
     || (["mouse", "pen"].includes(pointerType) && !isMobile)
   );
+  const usesDesktopTrailingSpacer = enabled && !isMobile;
   const rulerElement = enabled ? trackElement.closest?.(".timeline-board")?.querySelector?.(".timeline-ruler-canvas") : null;
   if (enabled) {
     trackElement.classList?.add("is-trimming");
@@ -79,7 +80,7 @@ export function createTimelineEdgeAutoScroller({ trackElement, pointerType, time
     spacerElement.style.width = `${geometry.width}px`;
   };
 
-  if (enabled && scrollElement?.ownerDocument?.createElement) {
+  if (usesDesktopTrailingSpacer && scrollElement?.ownerDocument?.createElement) {
     const previousSpacer = scrollElement.querySelector?.("[data-timeline-trim-scroll-spacer]");
     previousSpacer?.__timelineTrimCleanup?.();
     previousSpacer?.remove?.();

src/lib/timelineEdgeAutoScroll.test.js

@@ -66,6 +66,29 @@ describe("mobile timeline edge auto-scroll", () => {
     expect(rulerClasses.has("is-trimming")).toBe(false);
   });
 
+  it("never creates or settles a desktop trailing spacer on mobile", () => {
+    let appended = 0;
+    const scrollElement = {
+      clientWidth: 400,
+      scrollLeft: 120,
+      ownerDocument: { createElement: () => ({}) },
+      appendChild: () => { appended += 1; },
+    };
+    const track = {
+      classList: { add() {}, remove() {} },
+      closest: () => null,
+      parentElement: scrollElement,
+    };
+    const scroller = createTimelineEdgeAutoScroller({
+      trackElement: track,
+      pointerType: "touch",
+      win: { matchMedia: () => ({ matches: true }), requestAnimationFrame: () => 1, cancelAnimationFrame() {} },
+    });
+    scroller.stop();
+    expect(appended).toBe(0);
+    expect(scrollElement.scrollLeft).toBe(120);
+  });
+
   it("enables the same edge-scroll lifecycle for desktop mouse trimming", () => {
     const values = new Set();
     const track = {