Gradients-Graphics&Multimedia-Example Open GL causes bug the first time it is used

Description:

Clicking on open Gl will bug out the example's screen. Clicking outside of the widget will turn back the screen to normal.

Steps to reproduce:

1.Open the Gradients example.

2. Click on the open GL in the right lower section of example.

Expected behavior:

The example should use GL without any visual bug.
Observed behavior:

A visual bug is observed when clicking on open GL.

Special rules:

Clicking outside of the widget will bring the widget's screen back to normal.