What is going on with the layout of my PhpStorm?

I've skipped the last couple of PhpStorm updates because there's always something that breaks, but I recently had to update to 2026.2.3 and now my editor panels are all over the place! Panels that are meant to be pinned to the sides just expand to fill the screen if I close the main editor windows. The way the layout reorganises itself makes absolutely no sense. That never used to happen until this update.

Can anyone tell me how I disable whatever this annoying new behaviour is?