fix binding the root background pixmap in case of depth mismatch

src/picom.c

@@ -1158,14 +1158,43 @@
 		}
 		auto pixmap = x_get_root_back_pixmap(&ps->c, ps->atoms);
 		if (pixmap != XCB_NONE) {
+			xcb_get_geometry_reply_t *r = xcb_get_geometry_reply(
+			    ps->c.c, xcb_get_geometry(ps->c.c, pixmap), NULL);
+			if (!r) {
+				goto err;
+			}
+
+			// We used to assume that pixmaps pointed by the root background
+			// pixmap atoms are owned by the root window and have the same
+			// depth and hence the same visual that we can use to bind them.
+			// However, some applications break this assumption, e.g. the
+			// Xfce's desktop manager xfdesktop that sets the _XROOTPMAP_ID
+			// atom to a pixmap owned by it that seems to always have 32 bpp
+			// depth when the common root window's depth is 24 bpp. So use the
+			// root window's visual only if the root background pixmap's depth
+			// matches the root window's depth. Otherwise, find a suitable
+			// visual for the root background pixmap's depth and use it.
+			//
+			// We can't obtain a suitable visual for the root background
+			// pixmap the same way as the win_bind_pixmap function because it
+			// requires a window and we have only a pixmap. We also can't not
+			// bind the root background pixmap in case of depth mismatch
+			// because some options rely on it's content, e.g.
+			// transparent-clipping.
+			xcb_visualid_t visual =
+			    r->depth == ps->c.screen_info->root_depth
+			        ? ps->c.screen_info->root_visual
+			        : x_get_visual_for_depth(&ps->c, r->depth);
+			free(r);
+
 			ps->root_image = ps->backend_data->ops->bind_pixmap(
-			    ps->backend_data, pixmap,
-			    x_get_visual_info(&ps->c, ps->c.screen_info->root_visual), false);
+			    ps->backend_data, pixmap, x_get_visual_info(&ps->c, visual), false);
 			if (ps->root_image) {
 				ps->backend_data->ops->set_image_property(
 				    ps->backend_data, IMAGE_PROPERTY_EFFECTIVE_SIZE,
 				    ps->root_image, (int[]){ps->root_width, ps->root_height});
 			} else {
+			err:
 				log_error("Failed to bind root back pixmap");
 			}
 		}

src/render.c

@@ -604,28 +604,35 @@
 	bool fill = false;
 	xcb_pixmap_t pixmap = x_get_root_back_pixmap(&ps->c, ps->atoms);
 
-	// Make sure the pixmap we got is valid
-	if (pixmap && !x_validate_pixmap(&ps->c, pixmap)) {
-		pixmap = XCB_NONE;
+	xcb_get_geometry_reply_t *r;
+	if (pixmap) {
+		r = xcb_get_geometry_reply(ps->c.c, xcb_get_geometry(ps->c.c, pixmap), NULL);
 	}
 
 	// Create a pixmap if there isn't any
-	if (!pixmap) {
+	xcb_visualid_t visual;
+	if (!pixmap || !r) {
 		pixmap =
 		    x_create_pixmap(&ps->c, (uint8_t)ps->c.screen_info->root_depth, 1, 1);
 		if (pixmap == XCB_NONE) {
 			log_error("Failed to create pixmaps for root tile.");
 			return false;
 		}
+		visual = ps->c.screen_info->root_visual;
 		fill = true;
+	} else {
+		visual = r->depth == ps->c.screen_info->root_depth
+		             ? ps->c.screen_info->root_visual
+		             : x_get_visual_for_depth(&ps->c, r->depth);
+		free(r);
 	}
 
 	// Create Picture
 	xcb_render_create_picture_value_list_t pa = {
 	    .repeat = true,
 	};
 	ps->root_tile_paint.pict = x_create_picture_with_visual_and_pixmap(
-	    &ps->c, ps->c.screen_info->root_visual, pixmap, XCB_RENDER_CP_REPEAT, &pa);
+	    &ps->c, visual, pixmap, XCB_RENDER_CP_REPEAT, &pa);
 
 	// Fill pixmap if needed
 	if (fill) {
@@ -646,8 +653,7 @@
 	ps->root_tile_paint.pixmap = pixmap;
 #ifdef CONFIG_OPENGL
 	if (BKEND_GLX == ps->o.backend) {
-		return paint_bind_tex(ps, &ps->root_tile_paint, 0, 0, true, 0,
-		                      ps->c.screen_info->root_visual, false);
+		return paint_bind_tex(ps, &ps->root_tile_paint, 0, 0, true, 0, visual, false);
 	}
 #endif
 

src/x.c

@@ -321,6 +321,21 @@
 	return x_get_visual_for_pictfmt(g_pictfmts, pictfmt->id);
 }
 
+xcb_visualid_t x_get_visual_for_depth(struct x_connection *c, uint8_t depth) {
+	xcb_screen_iterator_t screen_it = xcb_setup_roots_iterator(xcb_get_setup(c->c));
+	for (; screen_it.rem; xcb_screen_next(&screen_it)) {
+		xcb_depth_iterator_t depth_it =
+		    xcb_screen_allowed_depths_iterator(screen_it.data);
+		for (; depth_it.rem; xcb_depth_next(&depth_it)) {
+			if (depth_it.data->depth == depth) {
+				return xcb_depth_visuals_iterator(depth_it.data).data->visual_id;
+			}
+		}
+	}
+
+	return XCB_NONE;
+}
+
 xcb_render_pictformat_t
 x_get_pictfmt_for_standard(struct x_connection *c, xcb_pict_standard_t std) {
 	x_get_server_pictfmts(c);
@@ -689,27 +704,6 @@
 	return XCB_NONE;
 }
 
-/**
- * Validate a pixmap.
- *
- * Detect whether the pixmap is valid with XGetGeometry. Well, maybe there
- * are better ways.
- */
-bool x_validate_pixmap(struct x_connection *c, xcb_pixmap_t pixmap) {
-	if (pixmap == XCB_NONE) {
-		return false;
-	}
-
-	auto r = xcb_get_geometry_reply(c->c, xcb_get_geometry(c->c, pixmap), NULL);
-	if (!r) {
-		return false;
-	}
-
-	bool ret = r->width && r->height;
-	free(r);
-	return ret;
-}
-
 /// We don't use the _XSETROOT_ID root window property as a source of the background
 /// pixmap because it most likely points to a dummy pixmap used to keep the colormap
 /// associated with the background pixmap alive but we listen for it's changes and update

src/x.h

@@ -356,8 +356,6 @@ const char *x_strerror(xcb_generic_error_t *e);
 
 xcb_pixmap_t x_create_pixmap(struct x_connection *, uint8_t depth, int width, int height);
 
-bool x_validate_pixmap(struct x_connection *, xcb_pixmap_t pxmap);
-
 /**
  * Free a <code>winprop_t</code>.
  *
@@ -408,6 +406,8 @@ struct xvisual_info x_get_visual_info(struct x_connection *c, xcb_visualid_t vis
 
 xcb_visualid_t x_get_visual_for_standard(struct x_connection *c, xcb_pict_standard_t std);
 
+xcb_visualid_t x_get_visual_for_depth(struct x_connection *c, uint8_t depth);
+
 xcb_render_pictformat_t
 x_get_pictfmt_for_standard(struct x_connection *c, xcb_pict_standard_t std);