Toggle fullscreen on double left mouse click in video area

[mrfoggg]

Currently, double-clicking with the left mouse button on the video playback area does not toggle fullscreen mode.

It would be very convenient if a double left mouse click on the video area toggled fullscreen on/off, similar to the behavior found in most video players (including MPV-based ones).

Why this is useful

• This is a widely accepted and expected UX pattern
• Improves usability with mouse-only navigation
• Matches behavior of common media players (mpv, VLC, Celluloid, web players, etc.)

Expected behavior

• Double left mouse click on the video area:
  • If windowed → switch to fullscreen
  • If fullscreen → exit fullscreen
• Should not interfere with single-click actions (play/pause, UI reveal)

Notes

• Ideally applies only to the video display area, not UI controls
• Could be optional via a setting, but enabled by default