Move Block example - White outline sometimes appear

Description:

White outlines of the squares may sometimes appear. They are very frequent if in fullscreen mode and relatively rare if in the default window size.

Steps to reproduce:

1. Open the Move block example.
2. It might take some time for you to see white lines appear
3. Enter fullscreen mode
4. You should see almost immediately many white lines

Expected behavior:

There should not be anything visible except the moving blue squares.
Observed behavior:

We can see some white lines which are in fact the outline of the squares.

Special notes: