render: whole-window transparency when unfocused

This adds a new option, colors.unfocused-alpha. When set to a value <
1.0, the entire window (*everything* - all bg/fg colors, images etc)
are made transparent while the window does not have keyboard focus.

The way this is implemented (trying to keep the patch small), we need
to do full-screen repaints while in this mode.

That is, all screen updates result in full-screen repaints when:

* The window does not have keyboard focus
* The window *just* gained keyboard focus (i.e. last frame was
  rendered with transparency)

As such, one can expect a fairly large performance impact when there’s
large amount of output while the window is unfocused.

Focused windows are unaffected, as are unfocused windows when
colors.unfocused-alpha == 1.0.

CHANGELOG.md

@@ -144,6 +144,7 @@
 * Mouse buttons 6/7 (mouse wheel left/right).
 * `url.uri-characters` option to `foot.ini`
   (https://codeberg.org/dnkl/foot/issues/654).
+* `colors.unfocused-alpha` to `foot.ini`.
 
 
 ### Changed

config.c

@@ -1359,6 +1359,18 @@ parse_section_colors(const char *key, const char *value, struct config *conf,
         return true;
     }
 
+    else if (strcmp(key, "unfocused-alpha") == 0) {
+        double alpha;
+        if (!str_to_double(value, &alpha) || alpha < 0. || alpha > 1.) {
+            LOG_AND_NOTIFY_ERR("%s:%d: [colors]: unfocused-alpha: expected a value in the range 0.0-1.0",
+                    path, lineno);
+            return false;
+        }
+
+        conf->colors.unfocused_alpha = alpha * 65535.;
+        return true;
+    }
+
     else {
         LOG_AND_NOTIFY_ERR("%s:%d: [colors]: %s: invalid key", path, lineno, key);
         return false;
@@ -2940,6 +2952,7 @@ config_load(struct config *conf, const char *conf_path,
             .fg = default_foreground,
             .bg = default_background,
             .alpha = 0xffff,
+            .unfocused_alpha = 0xffff,
             .selection_fg = 0x80000000,  /* Use default bg */
             .selection_bg = 0x80000000,  /* Use default fg */
             .use_custom = {

config.h

@@ -154,6 +154,7 @@ struct config {
         uint32_t bg;
         uint32_t table[256];
         uint16_t alpha;
+        uint16_t unfocused_alpha;
         uint32_t selection_fg;
         uint32_t selection_bg;
         uint32_t url;

doc/foot.ini.5.scd

@@ -509,6 +509,26 @@ _alpha_ option.
 	Background translucency. A value in the range 0.0-1.0, where 0.0
 	means completely transparent, and 1.0 is opaque. Default: _1.0_.
 
+*unfocused-alpha*
+	Window translucency applied when window does not have keyboard
+	focus. Unlike *alpha*, this option affects everything (all
+	background colors, foreground colors, images etc).
+	
+	Note that *alpha* is also applied.
+	
+	Be aware that there is a performance penalty of using this, as
+	damage tracking is disabled while the window is
+	unfocused. I.e. all screen updates will result in fullscreen
+	repaints. This is not an issue when the window content is static,
+	or updates at a slow pace (e.g. top), but can be noticeable with
+	high amounts of output. Performance is unaffected in focused
+	windows.
+	
+	Valid values are in the range 0.0-1.0, where 0.0 means completely
+	transparent, and 1.0 is opaque.
+	
+	Default: _1.0_
+
 *selection-foreground*, *selection-background*
 	Foreground (text) and background color to use in selected
 	text. Note that *both* options must be set, or the default will be

foot.ini

@@ -66,6 +66,7 @@
 
 [colors]
 # alpha=1.0
+# unfocused-alpha=1.0
 # foreground=dcdccc
 # background=111111
 # regular0=222222  # black

render.c

@@ -2366,6 +2366,9 @@ grid_render(struct terminal *term)
         term->render.last_buf->height != buf->height ||
         term->flash.active || term->render.was_flashing ||
         term->is_searching != term->render.was_searching ||
+        (term->conf->colors.unfocused_alpha < 0xffff && (
+            !term->kbd_focus ||
+            !term->render.was_focused)) ||
         term->render.margins)
     {
         force_full_repaint(term, buf);
@@ -2392,6 +2395,7 @@ grid_render(struct terminal *term)
     term->render.last_buf = buf;
     term->render.was_flashing = term->flash.active;
     term->render.was_searching = term->is_searching;
+    term->render.was_focused = term->kbd_focus;
 
     shm_addref(buf);
     buf->age = 0;
@@ -2677,6 +2681,22 @@ grid_render(struct terminal *term)
     xassert(buf->width % term->scale == 0);
     xassert(buf->height % term->scale == 0);
 
+    if (!term->kbd_focus && term->conf->colors.unfocused_alpha < 0xffff) {
+        pixman_color_t unfocused_alpha =
+            {0, 0, 0, term->conf->colors.unfocused_alpha};
+        pixman_image_t *transparent =
+            pixman_image_create_solid_fill(&unfocused_alpha);
+
+        pixman_image_composite32(
+            PIXMAN_OP_IN_REVERSE, transparent, NULL, buf->pix[0],
+            0, 0, 0, 0, 0, 0, buf->width, buf->height);
+
+        pixman_image_unref(transparent);
+
+        wl_surface_damage_buffer(
+            term->window->surface, 0, 0, buf->width, buf->height);
+    }
+
     wl_surface_attach(term->window->surface, buf->wl_buf, 0, 0);
     wl_surface_commit(term->window->surface);
 }

terminal.h

@@ -554,6 +554,7 @@ struct terminal {
         struct buffer *last_buf;     /* Buffer we rendered to last time */
         bool was_flashing;           /* Flash was active last time we rendered */
         bool was_searching;
+        bool was_focused;
 
         size_t search_glyph_offset;