Home Assistant Editor Screen Oversized

It appears that I’ve pressed a key combination by accident and have screwed the size of the panel to the right of the menu and made it seriously oversized.

If it’s not that, I wonder if it was something to do with clicking on the title of the card editing window to enlarge it or down to Firefox synchronising with my mobile?

Reducing the Firefox view to less than 100% doesn’t help.

Either way, could someone get my display back to being useful please?