fix(D.2b): Slice 2 — buttons inside a whole-window-Draggable frame get their Click

Visual gate 2 (user): the "Slots" toggle caption was visible but unclickable.

Root cause (UiRoot.OnMouseDown/OnMouseUp): a left-press on a non-drag-source
widget inside a whole-window-Draggable frame (the inventory window's IA-12
drag) set _windowDragTarget; OnMouseUp then early-returned before emitting the
Click. So the paperdoll Slots button (the first plain button inside the
draggable inventory frame) never received its click. Chat/toolbar buttons
escape this — their frames aren't whole-window-draggable.

Fix (toolkit, root cause not band-aid): add UiElement.HandlesClick (a virtual
opt-out parallel to IsDragSource); UiButton overrides it true; OnMouseDown
routes a HandlesClick press to the widget (like CapturesPointerDrag) instead of
the window-drag, so OnMouseUp emits the Click. 2 regression tests lock it
(HandlesClick widget in a Draggable frame emits Click; a plain one doesn't).

Build + full App suite green (596, +2).

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

src/AcDream.App/UI/UiButton.cs

@@ -75,6 +75,11 @@ public sealed class UiButton : UiElement
     /// procedurally, so the importer must not build the button's children as widgets.</summary>
     public override bool ConsumesDatChildren => true;
 
+    /// <summary>A button is interactive — it must receive its Click even inside a whole-window-Draggable
+    /// frame (e.g. the paperdoll "Slots" toggle in the inventory window), so it opts out of the
+    /// IA-12 whole-window-drag that would otherwise swallow the press.</summary>
+    public override bool HandlesClick => true;
+
     /// <summary>
     /// Returns the File id for the current <see cref="ActiveState"/>, falling back to
     /// the DirectState ("" key) if the named state is absent.

src/AcDream.App/UI/UiElement.cs

@@ -121,6 +121,15 @@ public abstract class UiElement
     /// false; overridden by drag sources (e.g. an occupied <see cref="UiItemSlot"/>).</summary>
     public virtual bool IsDragSource => false;
 
+    /// <summary>If true, a left-press on this element is handled BY the element (it receives the Click
+    /// on release) instead of being captured as a whole-window move on a Draggable ancestor. Set by
+    /// interactive leaf widgets (e.g. <see cref="UiButton"/>) so they stay clickable inside a
+    /// whole-window-Draggable frame like the inventory window — where, without this, the IA-12
+    /// whole-window-drag swallows the press and the Click is never emitted. Distinct from
+    /// <see cref="IsDragSource"/> (starts a drag-drop) and <see cref="CapturesPointerDrag"/> (a
+    /// self-driven interior drag such as text selection). Default false.</summary>
+    public virtual bool HandlesClick => false;
+
     /// <summary>Minimum size enforced while resizing.</summary>
     public float MinWidth { get; set; } = 40f;
     public float MinHeight { get; set; } = 40f;

src/AcDream.App/UI/UiRoot.cs

@@ -275,12 +275,13 @@ public sealed class UiRoot : UiElement
                 // the bar movable by its empty cells / chrome.
                 _dragCandidate = true;
             }
-            else if (target.CapturesPointerDrag)
+            else if (target.CapturesPointerDrag || target.HandlesClick)
             {
-                // The pressed widget owns interior drags (e.g. text selection):
-                // do NOT move the ancestor window. The already-dispatched MouseDown
-                // event + SetCapture(target) let the target drive its own drag via
-                // the MouseMove events it receives while captured.
+                // The pressed widget owns its pointer interaction — either an interior drag (e.g. text
+                // selection, CapturesPointerDrag) or a click it must receive (e.g. a UiButton,
+                // HandlesClick). Either way do NOT move the ancestor window. The already-dispatched
+                // MouseDown + SetCapture(target) let the target handle it; on release OnMouseUp emits
+                // the Click over the same element. (A HandlesClick widget is not a drag candidate.)
                 _dragCandidate = false;
             }
             else if (window.Draggable)