Played a few levels today from where I left off yesterday, and hello glitch :D I didn't notice any cursor slow-down before this one, but navigating the menu to get back to the level select did feel laggy and eventually got unresponsive. Level 4686.
Getting back to level select, that was also glitched. Going to other levels, even on other pages, did not get rid of the glitch.
Going back to the main menu did not fix the glitch. I guess this is why I got in the habit of power-cycling.
When loading a level in this state, some of the background is corrupted too:
Definitely some kind of overflow, but good luck finding where xP