fix(#5657): preserve workspace file-tree scroll position across re-render

renderFileTree() clears its scroll container with box.innerHTML='' and rebuilds
every row. Detaching the rows collapses scrollHeight, so the browser clamps
scrollTop to 0 — every folder expand/collapse, breadcrumb nav, refresh, and
hidden-files toggle that re-runs the renderer teleported the reader to the top of
a long tree, losing their place (the dominant interaction in the panel).

Fix: capture box.scrollTop before the innerHTML='' wipe and restore it after the
normal render tail (after _renderTreeItems repaints and the container is tall
again). A plain scrollTop restore is sufficient and correct here — expand/collapse
insert or remove rows BELOW the clicked disclosure, so the clicked row keeps its
offset from the top; no getBoundingClientRect anchor delta is needed (that's only
for the prepend-above case in the chat messages list). The reporter's data-path
anchor sketch is deliberately NOT used: _renderTreeItems rows carry no data-path,
so that selector would match nothing.

The two early-return paths (no-workspace hides the box; empty-dir has nothing to
scroll) are left untouched — they legitimately reset. The restore guards only the
normal render tail. loadDir's two-render sequence (initial + post-expanded-dirs
prefetch) is handled correctly: each renderFileTree() call independently captures
then restores its own pre-wipe scrollTop across the await.

Test: tests/test_issue5657_filetree_scroll_preserve.py — static guards that the
capture precedes the wipe and the restore follows _renderTreeItems (per the
maintainer test plan; drops the getBoundingClientRect anchor assertion since the
fix deliberately doesn't use it). Fail-without-fix verified; 355 workspace-tree
regression tests green.

Closes #5657.

static/ui.js

@@ -17212,7 +17212,18 @@ if(!S._expandedDirs) S._expandedDirs=new Set();
 if(!S._dirCache) S._dirCache={};
 
 function renderFileTree(){
-  const box=$('fileTree');box.innerHTML='';
+  const box=$('fileTree');
+  // #5657: capture the scroll position before wiping the container. box.innerHTML=''
+  // detaches every row, collapsing scrollHeight so the browser clamps scrollTop to 0;
+  // without this, every expand/collapse, breadcrumb nav, refresh, and hidden-files
+  // toggle that re-runs renderFileTree() teleports the reader back to the top of a
+  // long tree. Restored only after the normal render tail below — the two early-return
+  // paths (no-workspace hides the box; empty-dir has nothing to scroll) legitimately
+  // reset. A plain scrollTop restore suffices here: expand/collapse insert/remove rows
+  // BELOW the clicked disclosure, so the clicked row keeps its offset from the top (no
+  // getBoundingClientRect anchor delta needed — that's only for prepend-above cases).
+  const prevScrollTop=box?box.scrollTop:0;
+  box.innerHTML='';
   // Cache current dir entries
   S._dirCache[S.currentDir||'.']=S.entries;
   // Show empty-state when no workspace is set or the directory is empty (#703)
@@ -17231,6 +17242,8 @@ function renderFileTree(){
     return;
   }
   _renderTreeItems(box, visibleEntries, 0);
+  // #5657: restore the pre-wipe scroll position now that the tree is tall again.
+  if(box) box.scrollTop=prevScrollTop;
 }
 
 let _wsActiveDragPath=null;

tests/test_issue5657_filetree_scroll_preserve.py

@@ -0,0 +1,64 @@
+"""Regression coverage for #5657 — workspace file-tree preserves scroll on re-render.
+
+`renderFileTree()` clears its scroll container with ``box.innerHTML=''`` and
+rebuilds every row. That detaches the rows, collapses ``scrollHeight``, and the
+browser clamps ``scrollTop`` to 0 — so every folder expand/collapse, breadcrumb
+nav, refresh, and hidden-files toggle that re-runs the renderer teleported the
+reader to the top of a long tree.
+
+Fix: capture ``scrollTop`` before the wipe and restore it after the normal
+render tail. A plain scrollTop restore is sufficient (expand/collapse insert or
+remove rows BELOW the clicked disclosure, so the clicked row keeps its offset) —
+the reporter's getBoundingClientRect anchor sketch is deliberately NOT used, and
+the ``.file-item`` rows carry no ``data-path`` for it anyway.
+"""
+
+from pathlib import Path
+
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+UI_JS = (REPO_ROOT / "static" / "ui.js").read_text(encoding="utf-8")
+
+
+def _render_file_tree_body() -> str:
+    start = UI_JS.index("function renderFileTree()")
+    end = UI_JS.index("\nfunction ", start + 1)
+    return UI_JS[start:end]
+
+
+def _render_file_tree_code() -> str:
+    """Body with // comment lines stripped, so assertions anchor on real code."""
+    lines = [ln for ln in _render_file_tree_body().splitlines() if not ln.strip().startswith("//")]
+    return "\n".join(lines)
+
+
+def test_render_file_tree_captures_and_restores_scrolltop():
+    body = _render_file_tree_code()
+    # It must still wipe the container (the behavior that loses scroll)...
+    assert "box.innerHTML=''" in body or 'box.innerHTML = ""' in body
+    # ...and it must capture scrollTop BEFORE the wipe and restore it after.
+    assert "scrollTop" in body, (
+        "renderFileTree must capture + restore scrollTop around the innerHTML wipe (#5657)"
+    )
+    capture_idx = body.index("prevScrollTop=box")
+    wipe_idx = body.index("box.innerHTML=''")
+    assert capture_idx < wipe_idx, (
+        "scrollTop must be captured BEFORE box.innerHTML='' so it isn't already clamped to 0"
+    )
+    render_idx = body.index("_renderTreeItems(box")
+    restore_idx = body.rindex("box.scrollTop=")
+    assert restore_idx > render_idx, (
+        "scrollTop must be restored AFTER _renderTreeItems repaints the tree"
+    )
+
+
+def test_early_return_paths_still_reset_scroll():
+    # The no-workspace and empty-dir early returns legitimately want scroll reset
+    # (box hidden / nothing to scroll) — the restore must live on the normal tail
+    # only, i.e. AFTER _renderTreeItems, not before the early returns.
+    body = _render_file_tree_code()
+    render_idx = body.index("_renderTreeItems(box")
+    tail = body[render_idx:]
+    assert "box.scrollTop=" in tail or "box.scrollTop =" in tail, (
+        "the scrollTop restore must be on the normal render tail, after _renderTreeItems"
+    )