Window preview is cut to Windowed mode resolution in Borderless/Fullscreen mode

[Sinamer]

Describe the bug: Works correctly with --tk flag
Window preview always cuts to the size of the window in Windowed mode, when it should show the full window in Fullscreen or Borderless modes

Screenshots or videos showing encountered issue: https://streamable.com/171yp4

osu!lazer version: 2020.1204.0

Logs:
runtime.log

[peppy]

Looks like we are switching to windowed mode on focus loss when we shouldn't be.

[peppy]

Okay, so there's a bit more to this.

1. Borderless isn't working as it is on osuTK due to the missing one-pixel hack. This means that borderless would should correctly in the preview but it's actually just an alias of fullscreen right now.
2. Fullscreen is behaving more correctly in SDL than osuTK. In osuTK, alt-tabbing from osu! when it's running fullscreen will not restore the desktop to the normal desktop resolution, leaving osu! to run in a windowed (borderless) state at its own specified resolution.

If you check other games that run fullscreen, you'll probably find that they don't get a preview at all. The only reason we're getting one here is due to SDL seemingly switching back to the previous window size briefly on alt-tab. I've confirmed that this isn't something being done at our end. If we really cared enough about the preview being correctly sized, we would likely need to adjust the window size to match the proposed fullscreen size before switching to fullscreen.

Maybe something we do want to consider, but it's extra state to manage at our end.

[Sinamer]

This has been fixed, the game no longer changes to windowed mode on task switching, and the preview is sized correctly, displaying a black rectangle, as intended for exclusive fullscreen.

Clip.134.mp4