Oh, I recall something else! Considering the idea:
Last year I raised this issue, which got fixed. Considering one comment, I suppose that certain terminals may try to do some fancy decorating themselves, only for it to be sanitized and rendered as pitch black box. I can still see this in Nautilus running in a Fedora 40 qube, but it’s not as blatant as in that issue. I’ll attach it, so I can showcase something else, what’s less distracting than that one ugly box from the issue:
While the black box on the folder being dragged is there alright (I suppose Nautilus tries to draw a nice-looking shadow here), please open this attachment in a new tab and see, that this screenshot, which was taken in dom0 with a mountainous photograph wallpaper (/usr/share/backgrounds/qubes/benjamin-voros-phIFdC6lA4E-unsplash.jpg to be precise) has no snippets of this wallpaper at the outside of the xfwm4 window border, but instead there’s plain white color, not even the window shadows I have enabled.
I suppose that certain tools may just act differently than expected, but also I can’t tell, why your specific configuration renders the way it renders. We can try and debug it, but I’d start with some details like the i3 and picom versions used - are they taken straight from the Qubes dom0 repository/Fedora 37 repository, or some custom compilation is taking place here? Does it behave the same way on Fedora 37 with its/your custom i3 and picom config? Asking, since it may be related to this point:
Browsers/terminals/windows of any kind that are not dom0 have rounded corners, but the part of the corner that is removed is black rather than transparent.