From 899b768b744c74e88e54e6d8eb32f53accea79d8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Wed, 19 Jul 2023 16:34:42 +0200
Subject: [PATCH] =?UTF-8?q?render:=20disable=20transparency=20when=20we?=
 =?UTF-8?q?=E2=80=99re=20fullscreened?=
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The wayland protocol recommends (or mandates?) that compositors render
a black background behind fullscreened transparent windows. I.e. you
never see what’s _actually_ behind the window.

So, if you have a white, but semi-transparent background in foot,
it’ll be rendered in a shade of gray.

Given this, it’s better to simply disable transparency while we’re
fullscreened. That way, we at least get the "correct" background
color.

Closes #1416
---
 CHANGELOG.md |  3 +++
 render.c     | 31 +++++++++++++++++++++++++++++--
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5e051081..9958e293 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -51,8 +51,11 @@
   configuration) from the monitor we were most recently mapped on,
   instead of the one least recently.
 * Starlight theme (the default theme) updated to [V4][starlight-v4]
+* Background transparency (alpha) is now disabled in fullscreened
+  windows ([#1416][1416]).
 
 [starlight-v4]: https://github.com/CosmicToast/starlight/blob/v4/CHANGELOG.md#v4
+[1416]: https://codeberg.org/dnkl/foot/issues/1416
 
 
 ### Deprecated
diff --git a/render.c b/render.c
index 11149b16..7d7c6348 100644
--- a/render.c
+++ b/render.c
@@ -526,8 +526,35 @@ render_cell(struct terminal *term, pixman_image_t *pix,
             uint32_t swap = _fg;
             _fg = _bg;
             _bg = swap;
-        } else if (cell->attrs.bg_src == COLOR_DEFAULT)
-            alpha = term->colors.alpha;
+        }
+
+        else if (cell->attrs.bg_src == COLOR_DEFAULT) {
+            if (term->window->is_fullscreen) {
+                /*
+                 * Note: disable transparency when fullscreened.
+                 *
+                 * This is because the wayland protocol recommends
+                 * (mandates even?) the compositor render a black
+                 * background behind fullscreened transparent windows.
+                 *
+                 * In other words, transparency does not work when
+                 * fullscreened, in the sense that you don't see
+                 * what's behind the window.
+                 *
+                 * And if we keep our alpha channel, the background
+                 * color will just look weird. For example, if the
+                 * background color is white, and alpha is 0.5, then
+                 * the window will be drawn in a shade of gray while
+                 * fullscreened.
+                 *
+                 * By disabling the alpha channel, the window will at
+                 * least be rendered in the intended background color.
+                 */
+                xassert(alpha == 0xffff);
+            } else {
+                alpha = term->colors.alpha;
+            }
+        }
     }
 
     if (unlikely(is_selected && _fg == _bg)) {