Created attachment 115719 [details]
export SAL_DISABLE_NATIVE_ALPHA=1 (which is equivalent to what the gtk3 back end does which is why I was looking at this) and in writer in the status bar the page icons will not have their bottom borders.
Caolán McNamara committed a patch related to this issue.
It has been pushed to "master":
Resolves: tdf#91392 orig map for orig code paths, and new map for new path
It will be available in 5.0.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.