In Movie Studio Platinum 12, when the trimmer is undocked, the trimmer only shows the sound portion of a video. If you move the slider you can see the video but only when moving the slider. As soon as you stop moving the slider the video disappears again. Worked OK in 11 but broken again in version 12. The fix is to dock the trimmer. Still a little flaky but it works as you can see the video portion then.
I did a Google search on this problem and discovered that it happened in previous older versions of Movie Studio.