Keep the left rail's order when its open item is lifted out

Co-authored-by: Sameh Khamis <[email protected]>

src/ui/railEdgeScroll.js

@@ -61,9 +61,25 @@ export function installRailEdgeScroll(doc = globalThis.document) {
   // Vue rewrites a rail's class list when it folds its labels, so whether the
   // highlight is showing is kept here, and the classes are put back each time.
   const gliding = new WeakSet();
+
+  // Where a rail lifts its open item out to sit beside the panel's title, the
+  // items left in the rail follow on from it in turn, so the next one in line
+  // is the one directly under it and a step brings that one up. A rail that
+  // keeps its open item in place keeps its own order.
+  function reorder(rail, on) {
+    const slots = [...rail.children].filter((slot) => slot.querySelector?.('.ui-rail__item'));
+    const at = on ? slots.findIndex((slot) => slot.contains(on)) : -1;
+    const lifted = at >= 0 && win.getComputedStyle(slots[at]).position === 'absolute';
+    slots.forEach((slot, index) => {
+      const order = lifted ? String((index - at + slots.length) % slots.length) : '';
+      if (slot.style.order !== order) slot.style.order = order;
+    });
+  }
+
   function glide(rail) {
     rail.classList.toggle('ui-rail--edge-hot', rail === hot);
     const on = rail.querySelector('.ui-rail__item[aria-pressed="true"]');
+    reorder(rail, on);
     if (!on?.offsetWidth) {
       gliding.delete(rail);
       rail.classList.remove('ui-rail--glide');
@@ -245,8 +261,9 @@ export function installRailEdgeScroll(doc = globalThis.document) {
     doc.removeEventListener('pointerout', onLeave);
     win.removeEventListener('wheel', onWheel, { capture: true });
     win.removeEventListener('resize', onResize);
-    doc.querySelectorAll('.ui-rail').forEach((rail) =>
+    doc.querySelectorAll('.ui-rail').forEach((rail) => {
-      rail.classList.remove('ui-rail--glide', 'ui-rail--glide-jump', 'ui-rail--edge-hot'),
+      rail.classList.remove('ui-rail--glide', 'ui-rail--glide-jump', 'ui-rail--edge-hot');
-    );
+      reorder(rail, null);
+    });
   };
 }