This one is an interesting case. It was easier to simply switch
to AdwPreferencesPage than actually port all the GtkFrames in
there.
In addition to that, the mouse test page now uses a GtkPicture
instead of a GtkImage, and the GtkDrawingArea API changes are
reflected in the code.
This markup isn't needed since it's already added later in the function.
Also, it was ignored because of some dark magic with the comma operator
(which shouldn't have been there to begin with). That's why it wasn't
discovered earlier.
See https://en.cppreference.com/w/c/language/operator_other#Comma_operator
for some explanation on how the comma operator works.
