This corrects some alignments issues through new margins introduced in
93544b641b. Basic idea of this fix is to only add the new margins when
drawing a panel with background. These margins were added specifically
for the background boxes, so that makes sense.
Alternative fix to D13199.
This also fixes some margings added unintentionally in mentioned commit.
There is a little jump of the toolbar and the tabs in the Properties
when comparing the UI without this fix to 2.93:
{F12158085} {F12158039}
The jump is gone with this fix applied (compare to the 2.93 screenshot):
{F12158064}
While not a serious issue, this confirms that this fix actually tackles
the root of the issue.