feat: support transparency (RGBA) in i3bar (#3727)

We introduce a --transparency flag for i3bar in order to enable a mode which
supports the use of RGBA colors.

An important constraint here is that tray icons will always have
a fully transparent background.

fixes #3723

docs/userguide

@@ -1744,6 +1744,26 @@ bar {
 }
 --------------------------------------
 
+=== Transparency
+
+i3bar can support transparency by passing the +--transparency+ flag in the
+configuration:
+
+*Syntax*:
+--------------------------------------
+bar {
+    i3bar_command i3bar --transparency
+}
+--------------------------------------
+
+In the i3bar color configuration and i3bar status block color attribute you can
+then use colors in the RGBA format, i.e. the last two (hexadecimal) digits
+specify the opacity. For example, +#00000000+ will be completely transparent,
+while +#000000FF+ will be a fully opaque black (the same as +#000000+).
+
+Please note that due to the way the tray specification works, enabling this
+flag will cause all tray icons to have a transparent background.
+
 [[list_of_commands]]
 == List of commands
 

i3bar/include/configuration.h

@@ -48,6 +48,7 @@ typedef struct config_t {
 
     position_t position;
     bool verbose;
+    bool transparency;
     struct xcb_color_strings_t colors;
     bool disable_binding_mode_indicator;
     bool disable_ws;

i3bar/src/main.c

@@ -56,10 +56,11 @@ static char *expand_path(char *path) {
 }
 
 static void print_usage(char *elf_name) {
-    printf("Usage: %s -b bar_id [-s sock_path] [-h] [-v]\n", elf_name);
+    printf("Usage: %s -b bar_id [-s sock_path] [-t] [-h] [-v]\n", elf_name);
     printf("\n");
     printf("-b, --bar_id       <bar_id>\tBar ID for which to get the configuration\n");
     printf("-s, --socket       <sock_path>\tConnect to i3 via <sock_path>\n");
+    printf("-t, --transparency Enable transparency (RGBA colors)\n");
     printf("-h, --help         Display this help message and exit\n");
     printf("-v, --version      Display version number and exit\n");
     printf("-V, --verbose      Enable verbose mode\n");
@@ -105,12 +106,13 @@ int main(int argc, char **argv) {
     static struct option long_opt[] = {
         {"socket", required_argument, 0, 's'},
         {"bar_id", required_argument, 0, 'b'},
+        {"transparency", no_argument, 0, 't'},
         {"help", no_argument, 0, 'h'},
         {"version", no_argument, 0, 'v'},
         {"verbose", no_argument, 0, 'V'},
         {NULL, 0, 0, 0}};
 
-    while ((opt = getopt_long(argc, argv, "b:s:hvV", long_opt, &option_index)) != -1) {
+    while ((opt = getopt_long(argc, argv, "b:s:thvV", long_opt, &option_index)) != -1) {
         switch (opt) {
             case 's':
                 socket_path = expand_path(optarg);
@@ -122,6 +124,9 @@ int main(int argc, char **argv) {
             case 'b':
                 config.bar_id = sstrdup(optarg);
                 break;
+            case 't':
+                config.transparency = true;
+                break;
             case 'V':
                 config.verbose = true;
                 break;

i3bar/src/xcb.c

@@ -1197,7 +1197,21 @@ char *init_xcb_early(void) {
 
     depth = root_screen->root_depth;
     colormap = root_screen->default_colormap;
+    visual_type = config.transparency ? xcb_aux_find_visual_by_attrs(root_screen, -1, 32) : NULL;
+    if (visual_type != NULL) {
+        depth = xcb_aux_get_depth_of_visual(root_screen, visual_type->visual_id);
+        colormap = xcb_generate_id(xcb_connection);
+        xcb_void_cookie_t cm_cookie = xcb_create_colormap_checked(xcb_connection,
+                                                                  XCB_COLORMAP_ALLOC_NONE,
+                                                                  colormap,
+                                                                  xcb_root,
+                                                                  visual_type->visual_id);
+        if (xcb_request_failed(cm_cookie, "Could not allocate colormap")) {
+            exit(EXIT_FAILURE);
+        }
+    } else {
         visual_type = get_visualtype(root_screen);
+    }
 
     xcb_cursor_context_t *cursor_ctx;
     if (xcb_cursor_context_new(conn, root_screen, &cursor_ctx) == 0) {