Broken horizontal scrolling in Firefox

It has 2 manifestations:
- If you drag the bottom scrollbar and your cursor moves beneath the bottom of the grid, it will scroll vertically at the same time (and vice versa).
- If you have a cell selected when you start scrolling, the selection changes with your scrolling.

https://github.com/user-attachments/assets/bcc45d02-5c62-4627-b135-098a87292ac1

We've traced the issue to this old PR https://github.com/glideapps/glide-data-grid/pull/1063