Yeah, it happens in the web build
https://streamable.com/vm3h0w
It seems to be related to losing focus of the web player, so it might not be anything you can solve on your own. Not sure how much Godot exposes in that way. As a side note, I tried the Linux build, too, but the game wouldn't load properly. The intro sequence is just grey with some of the noise and it seems to stall out before it loads the main menu screen/the song.
Viewing post in Rick O'Shea jam comments
Oh wow, thanks for taking the time to test it out and take a video. If you have time, could you try it again, and hit the Esc key to bring up the Main menu, then hit Resume Game and see if it grabs the mouse again? I think this might be an itch.io-specific web bug and something I can maybe code around. Or at least create a key to allow you to regrab the mouse cursor focus.
I appreciate the info on the Linux build. That is a huge bummer, as I could not test that build. However my games in the past have worked on Linux as I usually have a friend test them. Your feedback gives me something to look into though. Thank you for taking the time to do that.