KTextEditor doesn't have a default icon pixmap set for any of the mark
types (including the predefined types such as MarkInterface::Warning).
Setting the pixmap manually makes the icon appear on the icon border.
Food for thought: Do we actually want border icons for warnings/errors/etc?
IMO there's enough visual "clutter" already, which makes you aware of problems in code. (Thinking of underlines, different bg color (optional), etc.)
For errors and warnings, I think it's a good idea to show the pixmap. But does this deal nicely with breakpoints? I.e. the breakpoint icon should always take precedence over the warning/error pixmaps
Looking at https://code.woboq.org/qt5/kf5/ktexteditor/src/view/kateviewhelpers.cpp.html#1669 it seems like error will be rendered over breakpoint. We could work around this by using a larger value than MarkInterface::Error for breakpoints?