itch.io

Nope, when I played it I was on a Windows PC running it in a browser (browser was Vivaldi). I had the game set to full screen when I played, but just tested again and have the same issue out of full screen. I actually think I can tell what's happening; My cursor leaves the game, even in full screen, for a moment, which causes the menu to close or stutter between open and closed. I can tell that's what's happening because my cursor briefly changes from your custom one to the standard OS cursor for a moment when the "stutter" happens. Because of this, I doubt this would be an issue testing it in the engine or on a desktop build. I think for the browser version you might be able to improve this  by increasing the size of the corner of the screen where mouse entry triggers the inventory opening? I'm not sure, though. You might also be able to just bypass the event that closes the menu if the cursor leaves the play area while the menu is open. It's a very nitpicky thing but this game is so cool that I hope you continue building similar stuff, and if you do that, it's probably worth it to fix even small problems like this.