Emulator

If the aspect ratio changes while a video is recording, recording will cause a 'you have resized the window while dumping frames, nothing can be done to handle this properly, your video will likely be broken' error message.
This wasn't an issue before, as dolphin always used exactly 4:3 or 16:9 aspect ratios and never change aspect unless you changed the force aspect ratio option. But now dolphin derives it's aspect ratio from VI timings and switches between 4:9 and 16:9 on the fly for gamecube games which means the aspect ratio isn't constant through the game.

Dolphin needs to pick one resolution to dump the video (probably specified by the user, as requested in #4773) and add black bars to the video when the aspect ratio doesn't match.

A workaround does exist for now, where you use the "Stretch to Window" aspect ratio option, after adjusting the window to the correct size (potentially by using the "auto resize window" option)