It probably needs this from Adwaita:

.titlebar:not(headerbar) {
  window.csd > & {
    // in csd we assume every titlebar is a headerbar so reset anything, this is needed for split toolbars cases
    padding: 0;
    background-color: transparent;
    background-image: none;
    border-style: none;
    border-color: transparent;
    box-shadow: none;
  }

  > separator { background-color: $borders_color; }  // FIXME: use darker border?

  @extend %titlebar;
}

Created attachment 359558 [details]
before

Created attachment 359559 [details]
after

That fragment does seem to improve things.

Oddly, gedit has a strange separator in its headerbar, both before and after. I've never noticed this on any other app I've run in HC (though I don't use it permanently).

not sure what this was about, or whether we should take it along for the ride:

(In reply to Daniel Boles from comment #1)
>   > separator { background-color: $borders_color; }  // FIXME: use darker
> border?

Created attachment 359560 [details] [review]
HighContrast: Fix titlebar containing >1 headerbar

by migrating the relevant code from Adwaita

--

This assumes the stuff from Adwaita is sufficient for all cases and that we do
not need to theme the separator specifically. I only tested quickly, so maybe
this isn’t 100%, but it does seem to resolve the reported problem.

See also https://bugzilla.gnome.org/show_bug.cgi?id=784562 where one of the patches looks, at first glance, like it may address this in a different way. I've not been able to test either set yet though

^ It doesn't look like that resolves this one.

(In reply to Daniel Boles from comment #3)
> Created attachment 359559 [details]
> after
> 
> That fragment does seem to improve things.

It looks safe to merge my patch over https://git.gnome.org/browse/gtk+/commit/?h=gtk-3-22&id=b4371728deda7041ec309a517e221b0e68768a94, as it does the same thing, except more of it. :)


> Oddly, gedit has a strange separator in its headerbar, both before and
> after. I've never noticed this on any other app I've run in HC (though I
> don't use it permanently).

filed as https://bugzilla.gnome.org/show_bug.cgi?id=788573

Created attachment 360996 [details] [review]
HighContrast: Fix titlebar containing >1 headerbar

by migrating the relevant code from Adwaita, to dodge unwanted doubling
up of the bottom border and such.

It also hopefully still encompasses whatever commit
b4371728deda7041ec309a517e221b0e68768a94 was trying to do; certainly, it
retains the resolution of the main bug/patch that one was attached with.

Attachment 360996 [details] pushed as d9df5df - HighContrast: Fix titlebar containing >1 headerbar