fix(studio): Task 3 review — UiRoot.Pick, RenderStack IDisposable, dt cleanup

Code-review follow-ups to the ImGui inspector:
- Add public UiRoot.Pick(x,y) over the private HitTestTopDown (honors
  Z-order + modal exclusivity); StudioWindow uses it instead of a manual
  UiElement.HitTest with subtracted ScreenPosition.
- RenderStack : IDisposable — disposes the GL pieces it owns in one place;
  StudioWindow OnClosing + Dispose both call _stack?.Dispose(), closing the
  error-path leak (only UiHost was disposed on the Dispose-without-OnClosing
  path).
- Drop the stale _dt field; OnRender passes its own dt to Tick + BeginFrame.
- Fix a stale PanelFbo comment.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/RenderBootstrap.cs

@@ -22,8 +22,21 @@ public sealed record RenderStack(
     Wb.WbDrawDispatcher DrawDispatcher,
     SceneLightingUboBinding LightingUbo,
     AcDream.App.UI.UiHost UiHost,
-    AcDream.App.UI.UiDatFont? VitalsDatFont)
+    AcDream.App.UI.UiDatFont? VitalsDatFont) : System.IDisposable
 {
+    /// <summary>Dispose the GL pieces this stack OWNS (everything created in
+    /// <see cref="RenderBootstrap.Create"/>). <see cref="Dats"/> + <see cref="Gl"/> are caller-owned
+    /// and NOT disposed here. Called once at studio teardown.</summary>
+    public void Dispose()
+    {
+        DrawDispatcher.Dispose();
+        MeshAdapter.Dispose();
+        TextureCache.Dispose();
+        MeshShader.Dispose();
+        LightingUbo.Dispose();
+        UiHost.Dispose();
+    }
+
     /// <summary>
     /// Resolves a sprite id (0x06xxxxxx) to a (GL handle, width, height) triple.
     /// Copied verbatim from GameWindow's ResolveChrome closure — it calls

src/AcDream.App/Studio/PanelFbo.cs

@@ -56,7 +56,7 @@ public sealed unsafe class PanelFbo : IDisposable
         _gl.BindFramebuffer(FramebufferTarget.Framebuffer, _fbo);
         _gl.Viewport(0, 0, (uint)width, (uint)height);
         _gl.Disable(EnableCap.ScissorTest);
-        _gl.ClearColor(0.18f, 0.18f, 0.18f, 1f);   // dark bg matching the old direct draw
+        _gl.ClearColor(0.18f, 0.18f, 0.18f, 1f);   // opaque dark-grey canvas background (the FBO IS the canvas)
         _gl.ClearDepth(1.0);
         _gl.DepthMask(true);
         _gl.Clear(ClearBufferMask.ColorBufferBit | ClearBufferMask.DepthBufferBit);

src/AcDream.App/Studio/StudioWindow.cs

@@ -24,7 +24,7 @@ namespace AcDream.App.Studio;
 ///   <item>Tree pane — the element hierarchy; click-to-select.</item>
 ///   <item>Properties pane — geometry/anchors/flags of the selected element.</item>
 ///   <item>Click-to-inspect — a left-click in the canvas selects the topmost
-///     element under the cursor via <see cref="UiRoot.HitTest"/>.</item>
+///     element under the cursor via <see cref="UiRoot.Pick"/>.</item>
 /// </list>
 ///
 /// The window is intentionally thin: no game world, no physics, no streaming —
@@ -127,12 +127,7 @@ public sealed class StudioWindow : IDisposable
         _inspector = new StudioInspector();
     }
 
-    private double _dt;
-
-    private void OnUpdate(double dt)
-    {
-        _dt = dt;
-    }
+    private void OnUpdate(double dt) { }
 
     private void OnRender(double dt)
     {
@@ -143,8 +138,8 @@ public sealed class StudioWindow : IDisposable
         int w  = _window!.Size.X;
         int h  = _window!.Size.Y;
 
-        // 1. Tick the UI widgets.
-        _stack.UiHost.Tick(_dt);
+        // 1. Tick the UI widgets (OnRender's own dt — Update + Render fire with the same delta).
+        _stack.UiHost.Tick(dt);
 
         // 2. Render the panel into the off-screen FBO; get the color texture.
         //    The FBO is the same logical size as the window, so element rects map 1:1 to
@@ -163,13 +158,12 @@ public sealed class StudioWindow : IDisposable
         if (panelTex != 0)
             click = _inspector.DrawCanvas((nint)panelTex, w, h);
 
-        // 6. If the user clicked inside the canvas, hit-test the panel tree and select.
-        if (click is { } c && _panelRoot is not null)
+        // 6. If the user clicked inside the canvas, hit-test the whole UI tree (UiRoot.Pick honors
+        //    Z-order + modal exclusivity) and select the topmost element. The canvas click coord is
+        //    in the same root-space the FBO was drawn in, so it maps 1:1.
+        if (click is { } c)
         {
-            // UiElement.HitTest is internal — accessible from AcDream.App.Studio (same assembly).
-            // We call it on the panel root with coords relative to its top-left.
-            var screenPos = _panelRoot.ScreenPosition;
-            var hit = _panelRoot.HitTest(c.x - screenPos.X, c.y - screenPos.Y);
+            var hit = _stack.UiHost.Root.Pick(c.x, c.y);
             if (hit is not null)
                 _inspector.Selected = hit;
         }
@@ -191,12 +185,7 @@ public sealed class StudioWindow : IDisposable
         _panelFbo?.Dispose();
         _imgui    = null;
         _panelFbo = null;
-        _stack?.DrawDispatcher.Dispose();
-        _stack?.MeshAdapter.Dispose();
-        _stack?.TextureCache.Dispose();
-        _stack?.MeshShader.Dispose();
-        _stack?.LightingUbo.Dispose();
-        _stack?.UiHost.Dispose();
+        _stack?.Dispose();      // whole render stack: dispatcher/mesh/textures/shader/ubo/uihost
         _dats?.Dispose();
         _dats  = null;
         _stack = null;
@@ -210,8 +199,9 @@ public sealed class StudioWindow : IDisposable
         _panelFbo = null;
         _window?.Dispose();
         _window = null;
-        // If OnClosing wasn't called (e.g. exception before Run()), clean up anyway.
-        _stack?.UiHost.Dispose();
+        // If OnClosing wasn't called (e.g. an exception before Run() completed), dispose the FULL
+        // stack anyway — the review flagged that disposing only UiHost here leaked the rest.
+        _stack?.Dispose();
         _dats?.Dispose();
         _dats  = null;
         _stack = null;

src/AcDream.App/UI/UiRoot.cs

@@ -655,6 +655,10 @@ public sealed class UiRoot : UiElement
         return (null, 0, 0);
     }
 
+    /// <summary>Public hit-test for tooling (the UI Studio inspector): the topmost element under
+    /// (x,y) in root space, honoring modal exclusivity + Z-order. Wraps the private HitTestTopDown.</summary>
+    public UiElement? Pick(int x, int y) => HitTestTopDown(x, y).element;
+
     private static UiElement? FindWindow(UiElement? e)
     {
         while (e is not null)