Region ("selection") background color doesn't show

Question

Like most Emacs users I have a setup!

But there's one thing I try not to set : faces etc. For one thing I'm not that good with color intuitions. Another: all previous experiences of doing this show it breaks in some corner cases.

But now I find that the region face is so non-different from the rest (default) that it effectively (visually) does not exist. (Region behavior ok just color change absent)

Note that with -r (--reverse-video) a dark blue -- not optimal but at least visible -- appears.

But with plain default startup it's utterly invisible.

Other than explicitly setting the region face, is there some option to make the region visible? [And is this some kind of regression? I've never had this before]

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Emacs started with -Q version 27.1, on ubuntu

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Added in response to Nickd.

Default color

Foreground: black
DistantForeground: unspecified
Background: white

Region color

Foreground: unspecified
DistantForeground: gtk_selection_fg_color
Background: gtk_selection_bg_color

Searching around there does seem to be some bug:

https://lists.gnu.org/archive/html/bug-gnu-emacs/2017-09/msg01098.html

And, reading between the lines, it seems the response is:

Bug : yes
Response: Wont fix

Oh Well 🙃

Answer 1

Looking at customize-face -> region it talks about gtk_selection_bg_color. Indeed I get a better visible selection by selecting another theme in what the xfce4-setttings-manager calls Appearance.

Yet the nearly invisible selection background is no problem in other applications, so they probably ignore or override the theme in one of the gazillion ways this is possible in gtk.

I can put a

*:selected {
  background: red;
}

into ~/.config/gtk-3.0/gtk.css and a freshly started thunderbird uses red as the background when selecting text in an email. And emacs now uses red too.

While I agree with the OP that changing such colors gets one easily into a mess, in this case it is probably saner to just fix the region background color manually with customize-face rather than messing with the css. Except using another Appearance theme is an option.

(This is with emacs 29 on Ubuntu 20.04 using xfce4.)