This is a result of not clearing the canvas used by the HUD before drawing into it.
Created attachment 157853 [details]
Comment on attachment 157853 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=157853&action=review
R=me, with one robustness suggestion.
> + if (canvasSize.fWidth != bounds().width() || canvasSize.fHeight != bounds().height())
If bounds() are empty the first time through this function, it'll crash on a null m_hudCanvas below. Maybe add a || !m_hudCanvas for safety?
Committed r125445: <http://trac.webkit.org/changeset/125445>