bug#65042: 30.0.50; alpha-background doesn't effect background of stipple in emacs 30.0.50 pgtk

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Felix <felix.dick@web.de> writes:

> Po Lu <luangruo@yahoo.com> writes:
>
>> Felix <felix.dick@web.de> writes:
>>
>>> The background of stipple stays opaque with alpha-background parameter
>>> set for transparency.
>>> This came up in the use of the new package 'indent-bars'.
>>> The github issue: https://github.com/jdtsmith/indent-bars/issues/8
>>> I tested this with 'emacs -Q' only enabling the needed packages (compat
>>> and indent-bars).
>>
>> Thanks.  Do stipples start displaying adequately with this change?
>>
> No it doesn't.
> But it gives weird artifacts (doubling the character one
> line above or something like that)

How about this?

diff --git a/src/pgtkterm.c b/src/pgtkterm.c
index 9c1fc7bef4e..a7c687d811d 100644
--- a/src/pgtkterm.c
+++ b/src/pgtkterm.c
@@ -1328,14 +1328,17 @@ fill_background_by_face (struct frame *f, struct face *face, int x, int y,
 			 int width, int height)
 {
   cairo_t *cr = pgtk_begin_cr_clip (f);
+  double r, g, b, a;
 
+  cairo_set_operator (cr, CAIRO_OPERATOR_SOURCE);
   cairo_rectangle (cr, x, y, width, height);
   cairo_clip (cr);
 
-  double r = ((face->background >> 16) & 0xff) / 255.0;
-  double g = ((face->background >> 8) & 0xff) / 255.0;
-  double b = ((face->background >> 0) & 0xff) / 255.0;
-  cairo_set_source_rgb (cr, r, g, b);
+  r = ((face->background >> 16) & 0xff) / 255.0;
+  g = ((face->background >> 8) & 0xff) / 255.0;
+  b = ((face->background >> 0) & 0xff) / 255.0;
+  a = f->alpha_background;
+  cairo_set_source_rgba (cr, r, g, b, a);
   cairo_paint (cr);
 
   if (face->stipple != 0)
@@ -1343,10 +1346,10 @@ fill_background_by_face (struct frame *f, struct face *face, int x, int y,
       cairo_pattern_t *mask
 	= FRAME_DISPLAY_INFO (f)->bitmaps[face->stipple - 1].pattern;
 
-      double r = ((face->foreground >> 16) & 0xff) / 255.0;
-      double g = ((face->foreground >> 8) & 0xff) / 255.0;
-      double b = ((face->foreground >> 0) & 0xff) / 255.0;
-      cairo_set_source_rgb (cr, r, g, b);
+      r = ((face->foreground >> 16) & 0xff) / 255.0;
+      g = ((face->foreground >> 8) & 0xff) / 255.0;
+      b = ((face->foreground >> 0) & 0xff) / 255.0;
+      cairo_set_source_rgba (cr, r, g, b, a);
       cairo_mask (cr, mask);
     }