Running `example/simple_example_1/2/` render issue on Linux

When running the example file simple_example_1/2/ in the example folder, the rendered popup window is just blank, while running the '` is showing the generated railway and animation correctly.

On Windows machine, everything seems to be fine.

Tried to play with the gl parameter, but no luck.

OS: Ubuntu 18.04
Flatland: newest from git.

Thanks @xzhaoma

We will look into this.

Best regards,

The Flatland Team