Windows Users. Hello Sir and/or Ma'am.
Am I being tricked or are you seeing the same thing in your 15.2.1 C1PRO (Version 2022)? This is the screenshot of the Viewer on my Dual Display layout. I tried resetting the toolbar and I think this is a bug and this is still exist in the forthcoming 15.3.X Beta 2. I think many of us here in the forums are helping with the beta program.
Anyways...I plan to bring this up via Support Case.
If you look over by the mouse placement, the "Flexible Space" appears as a vertical line, while the regular spacing element appears as a block. This oddity is only on the Viewer option, via Windows >> Viewer.
I recently updated to 15.2.1 from 12.5.1 and this oddity was the first time that jumped out. I tried rescaling the Viewer, minimizing, maximizing thinking it might be a screen refreshing issue. Nope.
Please let me know. If you confirm, please raise a case to Support so they will fix in the final release. This still persist in Beta 2 of the forthcoming 15.3.X.