Every title in Final Cut Pro has a preview thumbnail for Timeline. But standard titles have transparent backgrounds which is very convenient when we use them inside compound clips. On the contrary custom made titles are displayed with opaque thumbnails, even if those are PNG files with transparent background.
This is how a custom made title inside a compound clip is represented in the Timeline — there's a video under the title, but we can't see it because transparent background of the title is solid black in the preview.
The question is how to make really transparent background for large.png and small.png files inside a Motion Title template folder?