persist root window’s background contents to a pixmap

The commit title is fairly technical, so I’ll try to explain.

Recently, users of GDM3 (I’m sure) and LightDM (I think) have reported
that when switching to a new workspace, the contents of the previous
workspace are still visible. i3bar updates, though, so it is the X11
root window which is not being updated here.

When using GDM3, X11 will be started with -background none, and no
background pixmap or pixel is set. Then, apparently,
gnome-settings-daemon will display a fade animation from whatever is
currently on the window to the destination contents. I think this is to
avoid flickering when logging in, which would occur when just setting a
specific background pixmap or pixel.

So, this commit will, when i3 starts first (not on restarts), copy the
contents of the X11 root window (typicall a grey background, at least on
my machine with GDM3) into a pixmap and set that pixmap as background
pixmap. That way, the content will be preserved and one has a
background, instead of what is perceived as a bug :).

This commit has some chance of breakage, so I’m prepared to revert it
unless we can figure out the issues and roll forward.

src/main.c

@@ -794,6 +794,27 @@ int main(int argc, char *argv[]) {
     }
     xcb_ungrab_server(conn);
 
+    if (autostart) {
+        LOG("This is not an in-place restart, copying root window contents to a pixmap\n");
+        xcb_screen_t *root = xcb_aux_get_screen(conn, conn_screen);
+        uint16_t width = root->width_in_pixels;
+        uint16_t height = root->height_in_pixels;
+        xcb_pixmap_t pixmap = xcb_generate_id(conn);
+        xcb_gcontext_t gc = xcb_generate_id(conn);
+
+        xcb_create_pixmap(conn, root->root_depth, pixmap, root->root, width, height);
+
+        xcb_create_gc(conn, gc, root->root,
+            XCB_GC_FUNCTION | XCB_GC_PLANE_MASK | XCB_GC_FILL_STYLE | XCB_GC_SUBWINDOW_MODE,
+            (uint32_t[]){ XCB_GX_COPY, ~0, XCB_FILL_STYLE_SOLID, XCB_SUBWINDOW_MODE_INCLUDE_INFERIORS });
+
+        xcb_copy_area(conn, root->root, pixmap, gc, 0, 0, 0, 0, width, height);
+        xcb_change_window_attributes_checked(conn, root->root, XCB_CW_BACK_PIXMAP, (uint32_t[]){ pixmap });
+        xcb_flush(conn);
+        xcb_free_gc(conn, gc);
+        xcb_free_pixmap(conn, pixmap);
+    }
+
     struct sigaction action;
 
     action.sa_sigaction = handle_signal;