We very frequently move this in and out of the windows slot map on
`update_window_id` calls (and we call this a lot!). This alone showed up
as `memmove`s at roughly 1% perf in Instruments when scrolling a buffer
which makes sense, `Window` itself is 4kb in size. The fix is simple,
just box the `Window` instances, moving a pointer is cheap in
comparison.

Release Notes:

- N/A *or* Added/Fixed/Improved ...