Keep the panel rails still when a tall panel opens

A tall panel moves its stack to the focus or tail layout, and the rail
inside the stack went with it, so the Data Layers and Authoring buttons
jumped up and the right rail followed. Each layout pass now offsets the
rail back to the resting top, with the same transition as the stack, and
the right rail lines up with the left rail's buttons. Cyber pins the left
stack's top, so it flags that and the rail takes no offset.

src/ui/rightPanelRail.js

@@ -126,8 +126,11 @@ export function layoutRightPanelRail({
     viewportHeight,
     getComputedStyle,
   );
+  // Align with the left rail's buttons, which hold still while its stack moves.
+  const leftRailOffset =
+    parseFloat(leftStack?.style.getPropertyValue('--panel-rail-offset')) || 0;
   const alignedTop = Number.isFinite(leftStackTop)
-    ? leftStackTop
+    ? leftStackTop + leftRailOffset
     : viewportHeight * 0.26;
   const obstacleRects = [];