feat(D.2b): UI render infra — overlay layer, DrawFill, crisp text, write-mode focus

The retail-look render + focus primitives this chat pass builds on:

- TextRenderer: an OVERLAY layer (sprite/rect/text buckets flushed AFTER the
  normal layer) so an open popup composites on top of everything incl. rect
  panel backgrounds; a DrawFill primitive (solid quad via a 1x1 white texture)
  routed through the SPRITE bucket so a panel background draws UNDER its text
  instead of being washed by the later rect bucket; and the text pass now
  disables SampleAlphaToCoverage + Multisample so glyph alpha edges aren't
  dithered into MSAA coverage (the "fuzzy text") — self-contained GL state
  per feedback_render_self_contained_gl_state.
- UiRenderContext.DrawStringDat: snap the line baseline to a whole pixel ONCE
  then add the integer per-glyph offset (retail DrawCharacter takes an int
  pen-Y + schar m_VerticalOffsetBefore) — fixes the "letters dip down" jitter
  at a fractional line origin. Outline pass is now opt-in (retail gates it per
  element via SetOutline; default off = crisp fill-only). Adds DrawFill +
  Begin/EndOverlayLayer.
- UiElement: OnDrawOverlay + DrawOverlays (second traversal), FindRoot (blur
  self), ResetAnchorCapture (re-baseline an anchored element after reflow).
- UiRoot: runs the overlay pass after the main tree; Tab/Enter focuses the
  DefaultTextInput (write-mode activation); a left click on a non-edit target
  blurs the focused input (exit write mode without submitting).

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

src/AcDream.App/UI/UiRoot.cs

@@ -44,6 +44,10 @@ public sealed class UiRoot : UiElement
     /// <summary>Widget currently receiving keyboard events.</summary>
     public UiElement? KeyboardFocus { get; private set; }
 
+    /// <summary>The edit control activated by Tab/Enter when nothing is focused — retail's
+    /// chat input "write mode" toggle. Set by the host once the chat window is built.</summary>
+    public UiElement? DefaultTextInput { get; set; }
+
     /// <summary>
     /// Single modal overlay; while set, mouse clicks outside its rect
     /// are ignored. Retail sets this via Device vtable +0x48.
@@ -131,6 +135,13 @@ public sealed class UiRoot : UiElement
         // Render children (panels) sorted by z-order — modal last so it
         // sits on top.
         DrawSelfAndChildren(ctx);
+        // Second pass: open popups/menus draw ON TOP of the whole tree (so e.g. the
+        // chat channel menu isn't greyed by the translucent chat panel that draws
+        // after it in the main pass). Routed to the renderer's overlay layer so it
+        // beats even rect backgrounds. Faithful to retail's root-level MakePopup.
+        ctx.BeginOverlayLayer();
+        DrawOverlays(ctx);
+        ctx.EndOverlayLayer();
     }
 
     // ── Input entry points (called from GameWindow's Silk.NET handlers) ──
@@ -200,12 +211,18 @@ public sealed class UiRoot : UiElement
         var (target, _, _) = HitTestTopDown(x, y);
         if (target is null)
         {
+            // Clicking the 3D world exits write mode (no submit) and returns control to
+            // the character — retail blurs the chat input on an outside click.
+            if (btn == UiMouseButton.Left) SetKeyboardFocus(null);
             WorldMouseFallThrough?.Invoke(btn, x, y, flags);
             return;
         }
 
-        // Set keyboard focus if target accepts it.
-        if (target.AcceptsFocus) SetKeyboardFocus(target);
+        // Keyboard focus follows a left click: the input bar (an edit control) takes
+        // focus = enters write mode; clicking anything else (chrome, Send, scrollbar,
+        // menu, another window) blurs the input = exits write mode WITHOUT submitting.
+        if (btn == UiMouseButton.Left)
+            SetKeyboardFocus(target.AcceptsFocus ? target : null);
 
         SetCapture(target);
 
@@ -355,6 +372,18 @@ public sealed class UiRoot : UiElement
 
     public void OnKeyDown(int vk, uint lparam = 0)
     {
+        // Nothing focused yet: Tab or Enter enters "write mode" by focusing the chat
+        // input (retail's chat-activation hotkeys). Consumed so the same press doesn't
+        // also fall through to a game hotkey.
+        if (KeyboardFocus is null && DefaultTextInput is not null
+            && (vk == (int)Silk.NET.Input.Key.Tab
+                || vk == (int)Silk.NET.Input.Key.Enter
+                || vk == (int)Silk.NET.Input.Key.KeypadEnter))
+        {
+            SetKeyboardFocus(DefaultTextInput);
+            return;
+        }
+
         // Focus widget first.
         if (KeyboardFocus is not null)
         {