In order to open the toolbox panel on the right in write mode, you can either click on the arrow button or on one of the tool buttons (clef, key signature etc.). However, closing the panel only works with the arrow button. That’s not a big deal but I often find myself clicking on the highlighted button again and nothing happens. I think it would be a nice small enhancement if a click on this button closed the panel as well.
Martin
Or a total pain if you’d meant to hit another button in the panel?!
Alternatively, use Cmd/Ctrl+9 to open and close this panel, and Cmd/Ctrl+7 and 8 to open the left and bottom panels respectively.
It’s not a total pain (sorry, I don’t get that comment) but I also don’t see any reason why you shouldn’t be able to close the panel this way too as many applications do. Of course, I can an do use the keyboard shortcuts. However, to have a quick look at the entries of a special tool, it’s faster to use the mouse for me, and in this case I click on the desired button and then have to scroll down to click on close button. Again, it’s not a big deal at all, I just thought adapting the usability here couldn’t hurt either.
This panel exists in all modes, not just Write mode. Write mode is the only place where it has separate buttons that can be clicked to open the panel.
Write mode is also the only place in which (I’m pretty sure) every single one of those panel clicks can be achieved by using a Shift+something popover. If you learn to use the popovers, you don’t need that panel in Write mode.
Having played with it a little, I don’t think I actively disagree that it would be a nice thing to have. It would be inconsistent with all of the other panels (e.g. 14 other panels across five modes) but, on the other hand, it is already inconsistent in that you can click the buttons to open this panel.
I’m a strong supporter of the OP’s idea. I’m so used to clicking the same button to open and close or activate and deactivate things in other programs that I find it annoying that this doesn’t work here. So, yeah, please enable this. Clicking on a button in the right toolbar should both open and close the respective panel.
Yes, exactly this. That’s the reason why I thought the buttons should close the corresponding panel too, similar to other applications. If the right panel could only be opened and closed with the arrow button it would be more consistent with the general usability (but probably not better). The limitation to opening the panels with the buttons feels like an unnecessary restriction. However, it’s just a minor thing and there are certainly more important things to implement.
Great to hear, I’m not the only one. 
I second the OP’s idea too — even if I seldom use those buttons (thanks to the popovers) I had this behavior and felt it was annoying.
We’ll look at implementing this in the next update.
That would be fantastic. Thanks Daniel.
Thank you so much for listening. 
Thank you for implementing this. Could you possibly add this behavior to the buttons next to the left panel of Engrave mode as well?
I’ll make a note of this and look into it at some point.