I adjusted the crop effect to make the rectangle cover exactly the area I want to hide. In preview it looked OK, except that the rectangle had a thin but visible darker border around it. I didn't try to hide the border, instead I exported the video. But then I found that in the exported video the rectangle (with border) had expanded well outside the mobile screen area (like in my original Pictures). In other words, for whatever reason, in this project it appears that the sequence preview is not WYSIWYG with respect to the exported video. Any ideas on what I'm doing wrong? Borate's solution does work, I wanted to figure out the crop method as well.