bug#45735: Adding a flat-button style for boxes

[Pedro Andres Aranda Gutierrez <paaguti@gmail.com>]

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Tags: patch

Create a flat-button style for boxes. It is based on the simple button
style but uses the background colour instead of the foreground colour
for the box.
This greatly simplifies the extension of Emacs themes to adapt them to
modern GUIs where the buttons are flat.

As an example, I like the mode-line to be a bit higher than the regular
mode line (à la doom-modeline, but not as big):

;; Definition of the styles without the patch:
;; I need to look up my theme and get the colours from it. For
;; nord-emacs it looks like this
;;
;; '(mode-line ((t (:inherit mode-line :background "#434C5E" :box
(:line-width 4 :color "#434C5E" )))))
;; '(mode-line-inactive ((t (:inherit mode-line-inactive :background
;; "#3B4252" :box (:line-width 4 :color "#3B4252" )))))

;; With the patch, I just need to indicate how many pixels higher (x 2)
;; I want the mode-line to be. I can reuse these settings across themes
;;
'(mode-line ((t (:inherit mode-line :box (:line-width 4 :style
flat-button)))))
'(mode-line-inactive ((t (:inherit mode-line-inactive :box (:line-width 4
:style flat-button )))))



In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20,
cairo version 1.16.0)
 of 2021-01-09 built on paag-EliteBook
Repository revision: 29c7f8c915c3889dfd5b25878aa0692f826cd38f
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Ubuntu 20.04.1 LTS

Configured using:
 'configure --build=x86_64-linux-gnu --prefix=/usr
 '--includedir=${prefix}/include' '--mandir=${prefix}/share/man'
 '--infodir=${prefix}/share/info' --sysconfdir=/etc --localstatedir=/var
 --disable-silent-rules '--libdir=${prefix}/lib/x86_64-linux-gnu'
 '--libexecdir=${prefix}/lib/x86_64-linux-gnu' --disable-maintainer-mode
 --disable-dependency-tracking --prefix=/usr --sharedstatedir=/var/lib
 --program-suffix=28 --with-modules=yes --with-x --with-x-toolkit=gtk3
 --without-xaw3d 'CFLAGS=-g -O2
 -fdebug-prefix-map=/home/paag/Devel/emacs/emacs=. -fstack-protector-strong
 -Wformat -Werror=format-security' 'CPPFLAGS=-Wdate-time
 -D_FORTIFY_SOURCE=2' 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro''

/Pedro A. Aranda
-- 
Fragen sind nicht da um beantwortet zu werden,
Fragen sind da um gestellt zu werden
Georg Kreisler

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diff --git a/src/xfaces.c b/src/xfaces.c
index b3b19a9cb2..6346706126 100644
--- a/src/xfaces.c
+++ b/src/xfaces.c
@@ -3293,7 +3293,7 @@ DEFUN ("internal-set-lisp-face-attribute", Finternal_set_lisp_face_attribute,
 		}
 	      else if (EQ (k, QCstyle))
 		{
-		  if (!EQ (v, Qpressed_button) && !EQ (v, Qreleased_button))
+		    if (!EQ (v, Qpressed_button) && !EQ (v, Qreleased_button) && !EQ(v, Qflat_button))
 		    break;
 		}
 	      else
@@ -6031,6 +6031,10 @@ realize_gui_face (struct face_cache *cache, Lisp_Object attrs[LFACE_VECTOR_SIZE]
 		face->box = FACE_RAISED_BOX;
 	      else if (EQ (value, Qpressed_button))
 		face->box = FACE_SUNKEN_BOX;
+	      else if (EQ (value, Qflat_button)) {
+		  face->box = FACE_SIMPLE_BOX;
+		  face->box_color = face->background;
+	      }
 	    }
 	}
     }
@@ -6919,6 +6923,7 @@ syms_of_xfaces (void)
   DEFSYM (Qwave, "wave");
   DEFSYM (Qreleased_button, "released-button");
   DEFSYM (Qpressed_button, "pressed-button");
+  DEFSYM (Qflat_button, "flat-button");
   DEFSYM (Qnormal, "normal");
   DEFSYM (Qextra_light, "extra-light");
   DEFSYM (Qlight, "light");