macOS 26: Titlebar tabs + native fullscreen has problems

[mitchellh]

When mixing macos-titlebar-style = tabs and native fullscreen, there are many problems:

1. Titlebar tabs appear offset from the left
2. Hovering over the menu brings up the traffic lights which looks odd
3. New tabs create visual glitches

The workaround for most of the mess is to exit fullscreen, navigating between tabs either via keyboard or mouse, then re-enter fullscreen. This is not a solution; this is a workaround.

The solution I think is to "undo" titlebar tabs upon entering native fullscreen. We don't have any machinery to hook into this right now but that's probably the right solution, since titlebar tabs match the native style in native.

Screenshot:

[mitchellh]

This was fixed by the various titlebar PRs that came in since 1.2.2. I'll backport them all to 1.2.3.