I am using Vegas Pro 13. I want to make a video using split-screen in which 3 videos taken on a phone in 1080x1920 (portrait) are placed side-by-side in a project set to 1920x1080 (landscape) a bit like the arrangement of the three colours of the French flag. However when I try to do this using the pan/crop tool, either the clip stays in place in the centre but a black mask moves left or right and covers it up, or I manage to move the clip to the left or right, but it disappears under a static black mask. How can I just move my clips to the left or right in the composition?