fix: repair pinned message navigation for large sessions (#187)

frontend/src/lib/components/content/MessageList.svelte

@@ -146,7 +146,10 @@
   function scrollToDisplayIndex(
     index: number,
     attempt: number = 0,
+    reqId: number = lastScrollRequest,
   ) {
+    if (reqId !== lastScrollRequest) return;
+
     const v = virtualizer.instance;
     if (!v) return;
 
@@ -157,28 +160,50 @@
       (virtualCount !== desiredCount || index >= virtualCount)
     ) {
       requestAnimationFrame(() => {
-        scrollToDisplayIndex(index, attempt + 1);
+        scrollToDisplayIndex(index, attempt + 1, reqId);
       });
       return;
     }
 
-    // TanStack's scrollToIndex may continuously re-seek
-    // in dynamic mode. Use one offset seek to avoid
-    // visible scroll "fight."
-    const offsetAndAlign =
-      v.getOffsetForIndex(index, "start");
-    if (offsetAndAlign) {
-      const [offset] = offsetAndAlign;
-      v.scrollToOffset(
-        Math.round(offset),
-        { align: "start" },
-      );
+    // If the item is already rendered (in the current virtual window),
+    // use its exact measured offset. Predecessor sizes are known so
+    // getOffsetForIndex is accurate.
+    const virtualItems = v.getVirtualItems();
+    const isRendered = virtualItems.some(
+      (vi) => vi.index === index,
+    );
+    if (isRendered) {
+      const offsetAndAlign =
+        v.getOffsetForIndex(index, "start");
+      if (offsetAndAlign) {
+        const [offset] = offsetAndAlign;
+        v.scrollToOffset(
+          Math.round(offset),
+          { align: "start" },
+        );
+      }
       return;
     }
 
-    // Item not yet measured — use scrollToIndex which will
-    // estimate and then correct once measured.
+    // Item not yet in render window. scrollToIndex scrolls to an
+    // estimated position, but TanStack's reconcile loop exits after
+    // 1 stable frame — before ResizeObserver measurements (delayed
+    // by bumpVersion's setTimeout(0)) have updated the offsets. The
+    // scroll stops at an estimated position rather than the real one.
+    //
+    // Retry in 2 frames: by then ResizeObserver + bumpVersion have
+    // fired, measurements are updated, and the next attempt either
+    // finds the item rendered (for an exact offset scroll) or repeats
+    // with a more accurate estimate. Limit to 10 render retries
+    // (~320 ms) to avoid looping forever.
     v.scrollToIndex(index, { align: "start" });
+    if (attempt < 15) {
+      requestAnimationFrame(() => {
+        requestAnimationFrame(() => {
+          scrollToDisplayIndex(index, attempt + 1, reqId);
+        });
+      });
+    }
   }
 
   function raf(): Promise<void> {
@@ -195,7 +220,7 @@
       const idx = ui.sortNewestFirst
         ? displayItemsAsc.length - 1 - idxAsc
         : idxAsc;
-      scrollToDisplayIndex(idx);
+      scrollToDisplayIndex(idx, 0, reqId);
       return;
     }
 
@@ -217,7 +242,7 @@
     const loadedIdx = ui.sortNewestFirst
       ? displayItemsAsc.length - 1 - loadedIdxAsc
       : loadedIdxAsc;
-    scrollToDisplayIndex(loadedIdx);
+    scrollToDisplayIndex(loadedIdx, 0, reqId);
   }
 
   export function scrollToOrdinal(ordinal: number) {

frontend/src/lib/virtual/createVirtualizer.svelte.ts

@@ -144,6 +144,16 @@ export function createVirtualizer(
         scrollEl.scrollTop = 0;
         return;
       }
+      // Don't override an active scrollToIndex reconcile loop.
+      // scrollState.index is non-null while TanStack is iterating
+      // toward the target index. Calling scrollToOffset here would
+      // reset scrollState.index to null, breaking the loop and
+      // leaving the viewport at the wrong position (observed as
+      // pinned-message navigation stopping mid-scroll in ascending
+      // sort when the target item is not yet rendered).
+      // scrollState is typed private but is a plain public field at runtime.
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      if ((instance as any).scrollState?.index != null) return;
       if (scrollEl.scrollTop > 0) {
         instance.scrollToOffset(scrollEl.scrollTop);
       }