VLC 3 changes screen brighness on an iMac 5K when an external screen is connected

[jollino]

Hello all,
this is a very odd issue but I hope someone with a similar configuration can reproduce it.

My machine is an iMac 5K 27" (2017), with 24 GB of RAM and a Radeon Pro 580 with 8 GB of VRAM. I also have an external screen connected via a USB C-HDMI adapter. The second screen's settings cannot be controlled from the Mac as it's a "dumb" Dell monitor.

When the second screen is connected, VLC 3.0.1 and the current nightly build of VLC 3.0.2 trigger some small brightness changes for the main screen as soon as the file is opening (i.e. when the window adjusts its size to match the video's) and again when the file stops playing. Moreover, while the file is playing, changing the screen brightness using the keyboard shortcuts is not as immediate as it normally is, but it's rather laggy.

This happens the same whether VLC's window is on the first or second screen. Disconnecting the second screen makes the issue go away.

I already tried resetting all settings but that didn't help. VLC 2 doesn't exhibit this behavior, nor indeed does any other video application. The messages don't seem to show anything related to the problem.

Can anyone with a similar configuration see if they can reproduce the issue? I'm not sure how to report it without any logs that may help figuring this out.

Thanks!

[tyvlvyt]

I have a same problem with my macOS Catalina Public Beta 1 and LG UltraFine 4k on MacBook Air 2018 (clamshell mode).

Screen brightness automatically dims as I open a video and make it full screen (non native full screen mode).