Fix wxChoice/wxComboBox dropdown rendering and add tests

- Fix empty clip region bug in wx.js clipRect function
- Add dropdown mouse interaction tests
- Add *.log to gitignore
- Make build script clean by default (use --no-clean to skip)
- Add wxGLCanvas canvas element fix documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docs/06-WXGLCANVAS-CANVAS-ELEMENT-FIX.md

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+# 06 - wxGLCanvas WASM Canvas Element Integration
+
+This document describes how to fix wxGLCanvas to render visibly in WASM by creating its own canvas element.
+
+## Problem
+
+wxGLCanvas currently hardcodes `m_canvasTarget = "#canvas"` which creates WebGL context on the main wxUniversal 2D canvas, causing a conflict. The GL rendering doesn't appear because it's overwritten by 2D UI rendering.
+
+## Key Discovery
+
+The WASM port already supports **multiple canvas elements per window**:
+- Main window uses `#canvas` for 2D rendering
+- Child windows can each get their own canvas element via `createWindow(id, needsCanvas=true)`
+- Windows are positioned absolutely with z-index layering
+- JavaScript (wx.js) manages canvas creation, positioning, and context stacking
+
+## How KiCad Uses wxGLCanvas
+
+```
+wxFrame (EDA_DRAW_FRAME)
+  └── EDA_DRAW_PANEL_GAL (wxScrolledCanvas)
+         └── OPENGL_GAL (inherits from wxGLCanvas)
+              - Sized to match parent
+              - Child window positioned within parent
+              - Has its own GL context
+```
+
+Key KiCad files:
+- `kicad/common/gal/opengl/opengl_gal.cpp` - GL rendering implementation
+- `kicad/common/draw_panel_gal.cpp` - Creates OPENGL_GAL as child: `new OPENGL_GAL(..., this, ...)`
+- `kicad/include/gal/hidpi_gl_canvas.h` - wxGLCanvas wrapper class
+
+---
+
+## Solution
+
+wxGLCanvas should integrate with the existing WASM window system:
+
+1. **Create its own canvas element** - Use `createWindow(id, needsCanvas=true)` in JavaScript
+2. **Create WebGL context on that canvas** - Use selector `#window-{id} canvas`
+3. **Handle positioning via window system** - `setWindowRect()` positions the canvas
+
+### Key JavaScript Functions (already exist in wx.js)
+
+```javascript
+createWindow(id, needsCanvas, isVisible, classList)
+setWindowRect(id, x, y, width, height)
+setWindowVisibility(id, isVisible)
+destroyWindow(id)
+```
+
+---
+
+## Implementation
+
+### Step 1: Modify Create() to create canvas element
+
+In `wxwidgets/src/wasm/glcanvas.cpp`:
+
+```cpp
+bool wxGLCanvas::Create(wxWindow *parent, ...)
+{
+    if ( !wxWindow::Create(parent, id, pos, size, style, name) )
+        return false;
+
+    // Create a window with canvas element in JavaScript
+    int cssId = GetCSSId();
+    EM_ASM({
+        createWindow($0, true, true, "glcanvas");
+    }, cssId);
+
+    // Position the canvas
+    wxPoint screenPos = GetScreenPosition();
+    wxSize clientSize = GetClientSize();
+    EM_ASM({
+        setWindowRect($0, $1, $2, $3, $4);
+    }, cssId, screenPos.x, screenPos.y, clientSize.GetWidth(), clientSize.GetHeight());
+
+    // Set dynamic canvas selector
+    m_canvasTarget = wxString::Format("#window-%d canvas", cssId).ToStdString();
+
+    return CreateWebGLContext(dispAttrs);
+}
+```
+
+### Step 2: Override DoSetSize for resize handling
+
+```cpp
+void wxGLCanvas::DoSetSize(int x, int y, int width, int height, int sizeFlags)
+{
+    wxWindow::DoSetSize(x, y, width, height, sizeFlags);
+
+    wxPoint screenPos = GetScreenPosition();
+    wxSize clientSize = GetClientSize();
+    EM_ASM({
+        setWindowRect($0, $1, $2, $3, $4);
+    }, GetCSSId(), screenPos.x, screenPos.y, clientSize.GetWidth(), clientSize.GetHeight());
+}
+```
+
+### Step 3: Handle visibility
+
+```cpp
+void wxGLCanvas::DoShow(bool show)
+{
+    wxWindow::DoShow(show);
+    EM_ASM({
+        setWindowVisibility($0, $1);
+    }, GetCSSId(), show);
+}
+```
+
+### Step 4: Clean up in destructor
+
+```cpp
+wxGLCanvas::~wxGLCanvas()
+{
+    if ( m_webglContext > 0 )
+    {
+        emscripten_webgl_destroy_context(m_webglContext);
+        m_webglContext = 0;
+    }
+    EM_ASM({
+        destroyWindow($0);
+    }, GetCSSId());
+}
+```
+
+---
+
+## Files to Modify
+
+| File | Changes |
+|------|---------|
+| `wxwidgets/src/wasm/glcanvas.cpp` | Main implementation - create own canvas, dynamic selector, resize/visibility handling |
+| `wxwidgets/include/wx/wasm/glcanvas.h` | Add DoSetSize, DoShow declarations if needed |
+
+---
+
+## Testing
+
+The existing test app in `tests/wasm-app/` with the OpenGL tab will verify:
+1. Canvas element is created in DOM
+2. WebGL context is on the correct canvas
+3. GL rendering appears in the canvas area
+4. Legacy GL emulation works (via `-sLEGACY_GL_EMULATION`)