feat(ui): unify retained window mounts

src/AcDream.App/UI/UiScrollable.cs

@@ -33,6 +33,25 @@ public sealed class UiScrollable
     /// <summary>True when the offset is at (or past) the bottom — used for bottom-pin.</summary>
     public bool AtEnd => _scrollY >= MaxScroll;
 
+    /// <summary>
+    /// Update the content/view extents and clamp the current offset. When
+    /// <paramref name="preserveEnd"/> is true, a view that was at the end before
+    /// the geometry change remains at the end afterward. User-driven scrollbar
+    /// changes move <see cref="ScrollY"/> off the end, so later layout passes
+    /// preserve that selected position instead of snapping it back.
+    /// </summary>
+    public void SetExtents(int contentHeight, int viewHeight, bool preserveEnd = false)
+    {
+        bool wasAtEnd = AtEnd;
+        ContentHeight = Math.Max(0, contentHeight);
+        ViewHeight = Math.Max(0, viewHeight);
+
+        if (preserveEnd && wasAtEnd)
+            ScrollToEnd();
+        else
+            SetScrollY(_scrollY);
+    }
+
     /// <summary>Set the offset, clamped to [0, MaxScroll] (SetScrollableXY clamp).</summary>
     public void SetScrollY(int y) => _scrollY = Math.Clamp(y, 0, MaxScroll);