Every pointer move over a caption or border recalculated the client
area and erased and repainted the window and all its controls, so a
dialog flickered while the pointer moved over its title. Only a caption
button changes there, and only the frame is drawn again for it.
Fixes#123
Assisted-by: Claude Code:claude-opus-5-5
AI-Scope: reproduction under Xvfb, the fix to hover_frame, the native_frame_hover test and fixture
