fix(frontend): clear programmatic scroll flag when a content-fits send is a no-op

frontend/src/lib/composables/createScrollAnchor.svelte.test.ts

@@ -587,4 +587,81 @@ describe("createScrollAnchor final content", () => {
     expect(scrollAnchor.showIndicator).toBe(true);
     scrollAnchor.cleanup();
   });
+
+  it("respects a no-intent scroll-away following a no-op send scroll", async () => {
+    vi.useFakeTimers();
+    const scrollContainer = document.createElement("div");
+    setScrollGeometry(scrollContainer, 0, 500, 500);
+    const scrollToSpy = vi.spyOn(scrollContainer, "scrollTo");
+    const scrollAnchor = createScrollAnchor({ indicatorDelayMs: 150 });
+    scrollAnchor.attach(scrollContainer);
+
+    await scrollAnchor.scrollOnce();
+    scrollToSpy.mockClear();
+
+    setScrollGeometry(scrollContainer, 200, 1_000, 500);
+    scrollAnchor.onUserScroll();
+
+    await scrollAnchor.revealFinalContentIfFollowing();
+    vi.advanceTimersByTime(150);
+
+    expect(scrollToSpy).not.toHaveBeenCalled();
+    expect(scrollAnchor.unseenCount).toBe(1);
+    expect(scrollAnchor.showIndicator).toBe(true);
+    scrollAnchor.cleanup();
+  });
+
+  describe("movedDuringScroll accumulator (multi-pass)", () => {
+    afterEach(() => {
+      vi.useRealTimers();
+      vi.restoreAllMocks();
+    });
+
+    it("pass-0 no-op then pass-1 real move keeps the flag raised (shields in-flight scroll)", async () => {
+      vi.useFakeTimers();
+      const el = document.createElement("div");
+      let calls = 0;
+      Object.defineProperties(el, {
+        scrollTop: { configurable: true, value: 0 },
+        clientHeight: { configurable: true, value: 500 },
+        scrollHeight: { configurable: true, get: () => (calls++ === 0 ? 500 : 1000) },
+      });
+      const spy = vi.spyOn(el, "scrollTo");
+      const a = createScrollAnchor({ indicatorDelayMs: 150 });
+      a.attach(el);
+      await a.scrollOnce();
+      spy.mockClear();
+      Object.defineProperty(el, "scrollTop", { configurable: true, value: 200 });
+      a.onUserScroll();
+      await a.revealFinalContentIfFollowing();
+      vi.advanceTimersByTime(150);
+      expect(spy).toHaveBeenCalled();
+      a.cleanup();
+    });
+
+    it("every-pass no-op clears the flag so a later no-intent scroll disengages follow", async () => {
+      vi.useFakeTimers();
+      const el = document.createElement("div");
+      Object.defineProperties(el, {
+        scrollTop: { configurable: true, value: 0 },
+        scrollHeight: { configurable: true, value: 500 },
+        clientHeight: { configurable: true, value: 500 },
+      });
+      const spy = vi.spyOn(el, "scrollTo");
+      const a = createScrollAnchor({ indicatorDelayMs: 150 });
+      a.attach(el);
+      await a.scrollOnce();
+      spy.mockClear();
+      Object.defineProperties(el, {
+        scrollTop: { configurable: true, value: 200 },
+        scrollHeight: { configurable: true, value: 1000 },
+      });
+      a.onUserScroll();
+      await a.revealFinalContentIfFollowing();
+      vi.advanceTimersByTime(150);
+      expect(spy).not.toHaveBeenCalled();
+      expect(a.unseenCount).toBe(1);
+      a.cleanup();
+    });
+  });
 });

frontend/src/lib/composables/createScrollAnchor.svelte.ts

@@ -274,6 +274,16 @@ export function createScrollAnchor(options: ScrollAnchorOptions = {}) {
     const prefersReducedMotion = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
     let performedScroll = false;
     let previousScrollHeight = -1;
+    // Sticky across passes: tracks whether *any* pass will actually move the
+    // container. A `scrollTo` whose target clamps to the current position is a
+    // no-op and fires no `scroll`/`scrollend` event, so nothing routes into
+    // `onUserScroll`'s near-bottom branch to clear `programmaticScrollActive` —
+    // the flag would stay stuck and later gate off the no-intent scroll-away
+    // branch (e.g. keyboard Tab focus into an off-screen link). Only keep the
+    // flag raised when this pass will really move; once any pass moves, leave
+    // the flag raised so the smooth scroll's in-flight events stay shielded
+    // (the near-bottom branch clears it once they settle).
+    let movedDuringScroll = false;
     for (let pass = 0; pass <= MAX_FINAL_REVEAL_RECONCILIATION_PASSES; pass++) {
       await tick();
       if (
@@ -289,12 +299,18 @@ export function createScrollAnchor(options: ScrollAnchorOptions = {}) {
       }
       previousScrollHeight = currentScrollHeight;
       performedScroll = true;
+      if (scrollContainer.scrollTop + scrollContainer.clientHeight < currentScrollHeight) {
+        movedDuringScroll = true;
+      }
       programmaticScrollActive = true;
       scrollContainer.scrollTo({
         top: currentScrollHeight,
         behavior: prefersReducedMotion ? "auto" : "smooth",
       });
     }
+    if (!movedDuringScroll) {
+      programmaticScrollActive = false;
+    }
     return performedScroll;
   }