Created attachment 166966 [details] Example Previews are flat images today looking unattached. While flat is modern in mobile UIs the good old bevel gives also a clue that this element offers no interaction. The example shows the current flat character style preview while above with a bevel using the Shadow and DarkShow colors of the Breeze color theme. In would be inverted in case of Breeze Dark and the upper border becomes light grey while the lower is white then.
Is this only with the kf5 VCL plugin? At a quick glance, it looks like the underlying cause may be the same as underlying bug 138010 which you've added as "See also". As described in bug 138010 comment 2, I also see borders for the character dialog when using either the Fusion theme or the demo patch at https://gerrit.libreoffice.org/c/core/+/105408 (but I'm not so sure whether this is the right way to go...). This may need further investigation, and maybe discussion with the Breeze theme authors.
My "demo" patch https://gerrit.libreoffice.org/c/core/+/105237
Scrollwindow should have a border, and does with gen and gtk3. But not kf5, so this is a bug and has been reported. *** This bug has been marked as a duplicate of bug 138010 ***