I've been investigating this a bit more and the behaviour of the tear off is a bit odd. Some items will work with a mouse click whilst other require a mouse click and then a strike of the enter key.
Couple examples of what I mean.

On this one everything on the list works with just a mouse click except for G'Mic which requires the enter key as well.

For this one the first four items on the list work on the mouse click. The other tools will only be selected if after clicking the item the cursor is left over the tool and enter is hit, if the cursor is moved away from the clicked tool and then enter is hit the tool will not be selected.
For the rest of the tear off menus it's a bit hit and miss as to what will work with a mouse click and what needs the enter key as well.