Turtleart doesn't fill offscreen surface area with content.

[greenfeld]

godiard spotted a "video corruption" issue with TurtleArt which likely is TurtleArt's fault.

According to jnettlet, Turtle art draws a rendering surface larger than the screen. but doesn't fill it with a background color beyond the dimensions of the screen. So the contents of offscreen video memory are seen once you scroll down or sideways past what already is rendered, which could look like many things depending on what is in video RAM at that point.

Reproduction:

1. Start Turtleart.
2. Using the scrollbars for the main canvas, scroll downward in the drawing area a significant amount.

Seen in OLPC 11.2.0 os14 on both XO-1 & XO-1.5.

[walter]

 http://git.sugarlabs.org/turtleart/mainline/commit/6af63b82066eaa3026815ea22b41db4bb6f7ae5b

I think the behaviour will now be as expected.