During window resize (by mouse for example), wrapped lines are not painted properly. What is actually drawn on screen during resize depends on used technology. With DirectWrite enabled and double buffering disabled, only first subline of a wrapped line will be drawn, while the rest of sublines will be completely black. With GDI enabled and double buffering enabled, first subline of a wrapped line will be multiplied and drawn on every other subline instead of actual subline.
I attached two screenshots that demonstrate mentioned problems.
I'm aware that it takes some time to fully repaint window and that the problems will disappear when you stop resizing, however the problem doesn't affect all lines equally (first 10 or so lines always draw properly for me) and according to observed pattern it actually looks like the background of sublines is not properly erased during repaint. Not to mention that the effect is very noticeable when using DirectWrite and it doesn't look very nice.