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