There is a known bug where you name a custom block “%” and click “OK”. Then you click “Cancel” and the blocks are grouped. And then you go to Costumes and back to Code. On Scratch, clicking any category will reset the bug.
But in Scratch Labs, nothing happens. You have to reload the editor.

You better rename the topic, this happens on regular scratch too! Just add in an extension and do the same.
After you close the “my blocks” window, go to the costumes tab. Then come back to the code tab – it will be scrolled to “Motion” and you can only drag in motion blocks. You can't scroll through the blocks or click on a category. Without extensions do the same - but you can use Scratch normally after going back to the code tab, unlike with extensions.

My browser / operating system: Windows 7, Firefox 87.0, No Flash version detected