Transform effect trims footage outside of the viewport
This may be a little difficult to explain, but here goes.
It appears that a transform effect trims the footage that it is transforming to the viewport. This is slightly unexpected, as the name of the effect is transform, not transform & trim. One would expect the trimming to the viewport to be done after the last effect in the effects stack is applied.
Now, why is this important?
Well, I am trying to stabilize a bit of footage by tracking a feature in the footage, and then applying the inverse position to a transform effect. Unfortunately the clip is a little offset to the side, so I applied yet another transform effect to try and center & enlarge the footage a bit better. Unfortunately, since the transform effect is trimming the footage outside of it's viewport, I can't get that bit of missing footage back....
I'll try and explain the problem with a video. In the video I am just applying a simple transform to push the footage to one side, and another transform to push the video back. On the video that is pushed back, you can see the unintended trim of the footage.