Keep Mask in Same Spot While Key-Framing Transform - Final Cut Pro
I recently did this (as an experiment) by keyframing the motion of the mask in the opposite direction to the keyframed motion of the transform. In my example someone sitting in a chair (in front of a greenscreen) appeared as if they rose up out of a barrel in some background footage: I keyframed a mask where the bottom edge of the mask started at their head and worked down to their waist; at the same time the keyframes for the transform motion moved the greenscreen footage of their body upwards. The end result was a bit messy, but the bottom edge of the mask stayed roughly level with the lip of the barrel as the superimposed footage moved upwards.
To key frame the changes into my four-cornered mask I started with a narrow mask where all four corners were just above the head and set a first keyframe. I then moved to the end frame and modified the mask so that the bottom two corners were now at waist level, and set the second keyframe. So the mask height grew over the course of the transform.