When mouse events are off, if handled by xterm, preventDefault should be called on the event

Repro:

1. Make sure mouse events are no on in `~/.vimrc`
2. Open vim README.md
3. Scroll down via wheel, 🐛 the page scrolls as well as being handled by xterm and moving the cursor

Seen in: https://github.com/xtermjs/xterm.js/issues/5373#issuecomment-3393829386