Hello,

This is a followup to bug#47026, where we made
gnus-summary-highlight-line and gnus-highlight-selected-summary apply
the highlighting face up to and including the final newline.

In this new patch, I went over the faces in gnus-summary-highlight and
added :extend t to all of them.  Even though most of them have no
background or underline set in the default theme, :extend's special
handling means that other themes will not need to set that property
explicitly.