macOS: always use overlay scrollbar, flash on mouseover to make it clickable

[danielwe]

Here's an idea to solve the remaining woes with the macOS legacy scrollbar (mostly the fact that it takes up space on the alternate screen). The idea is to just not use it and always force the overlay style. However, when the OS preferred style is legacy, the scrollbar should be clickable without using a scroll wheel or trackpad gestures (as far as I can tell, that's the only reason the legacy style is still around), so in this case we should flash the scrollbar when the mouse moves over it.

Going this route means we no longer have to worry about scrollbar-related reflow, size mismatch between the core surface and framebuffer, or different terminal sizes on the primary and alternate screens. It renders #9295 obsolete.

Here's an implementation: main...danielwe:ghostty:overlay_only. It looks as follows. Thoughts?

Screen.Recording.2025-11-16.at.14.18.53.mov

[mitchellh]

Approved, please PR.