Describe the bug
When any palette flyout is open (e.g. Meter) and you click a different palette
(e.g. Ornament), the new palette does not open. The first click closes the
current one, and a second click is needed to open the new one.

To Reproduce

1. Click any palette (e.g. Meter) — it opens.
2. Without clicking anywhere else, click a different palette (e.g. Ornament).
3. The first palette closes but the second one does not open.
4. Clicking again opens it correctly.

Expected behavior
Clicking a palette should always open it immediately, regardless of what is
currently open.

Root cause
In _loadPaletteButtonHandler (palette.js), onmouseover schedules a 400ms
timeout to call showPalette. The onclick handler does not cancel this
pending timeout before calling showPalette itself, causing a race where the
hover timeout fires _hideMenus and closes the palette that the click just opened.

Environment

• Browser: Chrome / any
• OS: any
• Version: master (latest)