So I create a horizontal line with the pen tool. Which is essentially a shape with two points. When I create a new mask, the mask sets itself to the size of the shape, which in the y-axis is 0. Therefore when I double click on the mask try to edit it's size/shape, I can't stretch it in the y-axis because those points are on top of each other. If I create a diagonal line, this isn't a problem because the bounding box is rectangular and I can select all 4 points. It's only an issue with vertical or horizontal lines.
The workaround I've found is if I paste a mask from another layer it will retain it's original shape and I can edit it. However this is annoying, and it seems like there should be an easier way.
Thanks David. Yes, a track matte absolutely would work. As would pasting a mask from another layer, as I mentioned. I suppose it's a small issue. It's just annoying that something as simple as applying a mask, in this one specific situation, is completely useless. And I'm wondering if anyone else finds this frustrating and has found a simple workaround.