I would guess it's something either with gtk, or with your theme. Being it a theme, then it might be possible to reproduce this in things like entries for a Subject, or in the gtk3-demo application. Also, if you select the text by mouse, and it is not highlighted, will simple Delete button press delete the supposed-to-be-highlighted text or not?

I'm not so sure.   Other applications (e.g., gnome-terminal, gedit, firefox, gtk3-demo) all work as expected.  The selection process seems to work, e.g., I can delete the selected text.  It's the highlighting that's broken.

One more observation.  It seems that it's broken only in some of evo's widgets.    For example, I can't highlight in the  actual email, but I can in the header as displayed in the mail UI (e.g., the subject)

It sounds like GtkHTML reads colors for selected text from incorrect key/place.

Following Milan's off-line suggestion, it turns out that the problem is reproducible in gtkhtml-editor-test.  Just type some text and try selecting it.

Tested and confirming, with the current git master of gtk+ (commit 681871d) the selection color is same as color of the usual text. I'll check more next week.

Created attachment 206001 [details] [review]
gtkhtml patch

for gtkhtml;

Let's use GtkStyleContext instead of GtkStyle. It seems to be the main reason of the issue, caused by recent changes in Gtk+. This change is compatible with 3.2.x gtk+ too, thus it's safe to use.

Created commit 48f02e9 in gtkhtml master (4.3.5+)