Linux Mint MATE “notifications area” – much bigger than it needs to be

A week or so ago, triggered by I don't know what (but presumably something I did by accident) the Notification Area of my taskbar has become 3 times the size that it needs to be, taking up valuable space from the Window List. See attached screengrab with some scrawled notes on it.

I can't seem to get it to take up less space. As you can see, it's mostly just empty grey.

Am I a complete doofus? (I'd much rather be asking some obscure question about some command line operation). Any suggestions welcome.

EDIT – some further notes

• if I right-click on the drag handle, I can see that it is NOT locked. I have the option to move it, which moves the whole Notification Area, remove it or lock it.
• right-clicking on the large grey expanse does nothing.
• if I drag the drag handle over to the right (hoping this would move the divider), then the whole Notification Area, including the grey expanse, moves to the right of the clock.

EDIT – I just removed it from the taskbar and then added it again, and it's gone back to normal. I'd swear i tried that already. So, I'm now uncertain what to do with this question, which has now gone into "unreproducible by myself, aka 'one of those things'" territory. I'll leave it up since if someone can come along and tell me what caused the bug then they deserve the bounty.

EDIT – the version is "Notification Area 1.2.1"