Implements multiple mouse event improvements for UI elements:
- Mouse enter/exit events (#140): on_enter, on_exit callbacks and
hovered property for all UIDrawable types (Frame, Caption, Sprite, Grid)
- Headless click events (#111): Track simulated mouse position for
automation testing in headless mode
- Mouse move events (#141): on_move callback fires continuously while
mouse is within element bounds
- Grid cell events (#142): on_cell_enter, on_cell_exit, on_cell_click
callbacks with cell coordinates (x, y), plus hovered_cell property
Includes comprehensive tests for all new functionality.
Closes#140, closes#111, closes#141, closes#142🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
