AS3: mask does not work if maskee is over certain pixel size?

Question

I have a mask i'm using for a continuous scroll type thingy, and notice that when my masked sprite gets past a certain pixel size in height (2878) the mask does not mask. Has anyone experienced this? is this a bug?

to reproduce:

create a sprite over 2878 px in height and apply mask, mask breaks

create a sprite 2877 px in height and apply mask, mask works

Answer 1

I can't verify if that is a hard limit, but there are a bunch of similar size limits for bitmaps in Flash that crop up in various areas. One potential solution would be to use the scrollRect property of your content display object. When you set scrollRect you are essentially creating a rectangular mask and I'm almost positive I've done it with 5000+ pixel wide sprites in the past.