System and IINA version:

• macOS 13.0
• IINA - Both 1.3.0 (132) and nightly@f8811b6

Expected behavior: When Firefox/Chromium is open on monitor in full screen, and IINA is in full screen on the other, there should be no momentarily blackening of any screens at any time.

Actual behavior: When I open Firefox full-screen on one monitor, and IINA full-screen on the other, the screen with Firefox will sometimes be blackened. Sometimes this occurs somewhat randomly when a key is pressed on the keyboard while I have the Firefox window open. But the most consistent way to get this to occur is by clicking in the Firefox screen and then moving the cursor to the IINA monitor, which usually (though not 100% of the time) causes the Firefox monitor to go black, until I either click on the IINA monitor or move my mouse back (usually), though the screen will un-black after a moment in many cases. The screen also sometimes will turn black when I make a keystroke in the Firefox screen without moving my cursor to the IINA screen. This often happens when I am typing in a text box on a webpage, especially when I press the space bar.

I have tried this using both Firefox 106.0.2 (current stable) and Firefox's latest nightly build as of the time of my writing this, with the same results. I also tried using Chromium 106, which seems susceptible to the same problem. With most other programs, this does not occur (e.g., with Safari and Apple Mail). I don't know why it occurs with Firefox and Chromium and not those other programs, but it clearly has something to do with IINA, because it occurs with multiple other programs on the other screen, and it does not happen with MPV, nor with any other video player I try (e.g., QuickTime and VLC).

This phenomenon only occurs when a video is playing, not when one is open but paused. This does occur even if the video and/or audio tracks are disabled. More than that, whenever there is a UI panel of some kind open in IINA (playlist, settings sidebar, about window, etc.) above the IINA full-screen window, in any way, then the phenomenon does not occur even while playback is actually active.

The phenomenon does not occur when there are two windows split-screen on the same screen. This includes when IINA and Firefox are split on halves of the same screen, or even in fact when IINA is on one window and two firefox windows are split-screen in full-screen mode on the other.

The issue occurs no matter what hardware acceleration settings are chosen in Firefox or IINA. It also occurs no matter what codec is used in the video played in IINA and no matter its framerate (including whether it has an integer or non-integer framerate).

Both the monitors are external non-HiDPI FHD screens attached to a headless Mac Studio, one via HDMI and the other via Thunderbolt/DisplayPort cable.

Also, none of this ever occurs when one application or the other is NOT full-screen. This occurs whether IINA is set to legacy or new full screen mode.

Steps to reproduce: See above.

• MPV does not have this problem.
  This does not occur with MPV, nor with any other player I have used (QuickTime, VLC).

How often does this happen?
Almost every time the described steps are taken, although it is slightly random, it seems, and definitely doesn't happen quite 100% of the time.