Use viewport size for fullscreen canvas sizing

[kfastov]

Summary

• Use viewport size (visualViewport/inner) instead of screen.* for fullscreen canvas sizing
• Recompute sizing after fullscreenchange to avoid early/stale innerWidth/innerHeight values
• Preserve windowed sizing baseline across fullscreen resize events to restore correct windowed size on exit

Issue

Fixes #26096

Testing

• Manual repro on macOS 26.2 (MacBook Pro 14-inch, 2021) + Chrome 143.0.7499.193
• No automated test (hardware/OS-specific)

[kfastov]

Just found a problem in my code, checking again to find better solution