test(central-ui): InstancePicker — closed-panel ArrowDown, Escape no-op; dispatcher-ordering and overflow notes

ArrowDown on a picker that has never been opened pins the deferred path where
_tree is still null when the keydown is handled: the panel opens in that render
pass and the roving tab stop lands on the first row in the next one. Escape on a
closed picker is pinned as a no-op.

Two notes, no behaviour change: FirstVisibleInstance records WHY it may trust
TreeView.IsExpanded (Blazor Server's single renderer sync context runs the render
plus the pending ExpandAll/CollapseAll before the next keydown is dispatched — an
await that genuinely yields inside OnAfterRenderAsync would break that), and the
stylesheet warns integrators that the absolutely-positioned panel is clipped by
any ancestor with overflow: hidden/auto.

The ArrowDown focus interop assertion is kept — it was stable across repeated
runs — with a comment marking the tabindex as the behavioural check.

Claude-Session: https://claude.ai/code/session_01NcMG6xXdsJbmD2wYAgkK29

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Shared/InstancePicker.razor.cs

@@ -281,6 +281,16 @@ public partial class InstancePicker
     /// see — a collapsed area's contents are skipped, matching what Enter appears to pick.
     /// With a filter active every branch is expanded, so this degrades to plain document
     /// order over the filtered forest.
+    ///
+    /// <para>
+    /// Reading <see cref="TreeView{TItem}.IsExpanded"/> is safe here because Blazor Server
+    /// dispatches every UI event on one renderer sync context: the render and the pending
+    /// <c>ExpandAll</c>/<c>CollapseAll</c> that <see cref="OnAfterRenderAsync"/> applies both
+    /// complete before the NEXT keydown is dispatched, so the tree's expansion state can never
+    /// be one keystroke stale. That would stop holding if the pending action ever became
+    /// genuinely async (an await that yields inside <see cref="OnAfterRenderAsync"/>); it would
+    /// then have to be awaited before Enter is allowed to read the tree.
+    /// </para>
     /// </summary>
     private Instance? FirstVisibleInstance() => FirstVisibleInstance(_roots);
 

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Shared/InstancePicker.razor.css

@@ -22,7 +22,12 @@
 }
 
 /* Dropdown panel anchored under the field (the wrapper is position-relative).
-   Scrolls once the site's tree outgrows 40% of the viewport. */
+   Scrolls once the site's tree outgrows 40% of the viewport.
+
+   INTEGRATORS: the panel is absolutely positioned INSIDE the picker's wrapper, so any
+   ancestor with `overflow: hidden`/`auto` (a card body, a scrolling toolbar, a table cell)
+   clips it — the tree appears truncated or invisible while the field still says it is open.
+   Place the picker outside such a container, or let that container overflow visibly. */
 .ip-panel {
     position: absolute;
     left: 0;

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Shared/InstancePickerTests.cs

@@ -332,11 +332,41 @@ public class InstancePickerTests : BunitContext
         var items = cut.FindAll("[role='treeitem']");
         Assert.Equal("0", items[0].GetAttribute("tabindex"));
         Assert.All(items.Skip(1), i => Assert.Equal("-1", i.GetAttribute("tabindex")));
+
+        // The tab stop above is the behavioural assertion; this is the belt-and-braces check
+        // that real browser focus was requested too. ElementReference.FocusAsync surfaces as a
+        // JS invocation under Loose mode — drop this line if a bUnit upgrade stops recording it.
         Assert.Contains(
             JSInterop.Invocations,
             i => i.Identifier.Contains("focus", StringComparison.OrdinalIgnoreCase));
     }
 
+    [Fact]
+    public void ArrowDown_OnAClosedPicker_OpensThePanel_AndFocusesTheFirstRow()
+    {
+        // The tree does not exist yet when the keydown is handled, so this pins the deferred
+        // path: the panel opens in this render pass and the focus lands in the next one.
+        var cut = RenderPicker();
+        Assert.Empty(cut.FindAll("[data-test='t-panel']"));
+
+        Press(cut, "ArrowDown");
+
+        Assert.Single(cut.FindAll("[data-test='t-panel']"));
+        Assert.Equal("0", cut.FindAll("[role='treeitem']")[0].GetAttribute("tabindex"));
+    }
+
+    [Fact]
+    public void Escape_WhenClosed_IsANoOp()
+    {
+        var cut = RenderPicker(selectedInstanceId: 8);
+        Assert.Equal("Zulu", InputValue(cut));
+
+        Press(cut, "Escape");
+
+        Assert.Empty(cut.FindAll("[data-test='t-panel']"));
+        Assert.Equal("Zulu", InputValue(cut));
+    }
+
     [Fact]
     public void FilterIsNotCarriedIntoSelection()
     {