Taking a look at the original toolbox (with no custom CSS), we have this
When editing the width of ".blocklyToolbox" using Developer console, it cuts off the blocks contained within the category.
(I would like to clarify that this was the unintended effect I mentioned in the previous post, but I have found out that this doesn't happen when I reinject Blockly by restarting the full page instead of doing a live reload)
So it was mainly a misunderstanding, my bad