I increased the Main Toolbar icon size to 36. It resulted in the Group Mode icons slightly overlapping:
this isn’t ideal. 
Yep. I can duplicate it. It only occurs with these icons in the main toolbar though. I’m guessing it might be because these are hard coded into the Main Toolbar and are positioned next to each other with no spaces so they overlap above 18px
I, too, came across this today. Came here to report and found this existing bug report. The overlap appears to occur when the icon size for the Main Toolbar is increased to 50% or more of the toolbar height.
Format Toolbar seems to be fine. As @ownedbycats , @CarlM mentioned, isolated to the Group Mode icons only - they are treated as a single object within the toolbar customization.
Hi,
I think this may be a user interface bug.
Using Windows 11, Scrivener 64-bit 3.1.5.1 (2073405):
The Group mode icons appear as normal with a Main Toolbar icon size of 24 px.
Adjusting the icon size above 24 px under File > Options… > Appearance > General Interface > Main Toolbar causes the icons to be cut off in the interface, even with a generous Main Toolbar Height of 40 px.
I would attach a screenshot, but as a first time poster, I am not permitted so here is a link
That’s correct, they tend to disappear to the right. More so each time you increase the icons size.
That said, I usually run my setup at height 30 px and icon size 22 px, and even then, the icons are off-centre, but kind of 3D.
@Kevitec57 Thank you for verifying.
It sounds like this is not a new issue.
I hope the software team can fix it.
I did see this bug from April 2021, before I posted this message a few days ago.
I thought that my discovery might be a regression. I did not realize that it was still active three years later.
Yes, alas it is still active. You can check the release notes page to see if something should be fixed but appears to have regressed.