If true then the background, set with -background tag option, will be indented according to the -lmargin1, and -lmargin2 tag options. This has the (intended) side effect that hovering the margin (left from indented text) will not trigger events.
Depending on the widget content it can be very useful that also the background region will be indented. It's a trivial, but effective feature. See this example from application Scidb:
-indentbackground off | -indentbackground on | |