It doesn't always happen, but sometimes when you close the browser the following X error is caught by the GDK X error handler:
(WebKitWebProcess:3560): Gdk-ERROR **: The program 'WebKitWebProcess' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadDrawable (invalid Pixmap or Window parameter)'.
(Details: serial 50604 error_code 9 request_code 14 (core protocol) minor_code 0)
(Note to programmers: normally, X errors are reported asynchronously;
that is, you will receive the error a while after causing it.
To debug your program, run it with the GDK_SYNCHRONIZE environment
variable to change this behavior. You can then get a meaningful
backtrace from your debugger if you break on the gdk_x_error() function.)
This has also been fixed by r202040