The main window video display isn’t all that useful since it only ever shows the input video. All the options from single, split, side-by-side and compare only ever show the original. The preview button then opens a new view that calculates the preview.
I’d suggest saving the preview output to a temporary location and display it on the main view where available. Furthermore it would be nice if the application automatically calculates the preview just for the current frame.
You can probably can get rid of the preview view altogether and use the main view for displaying it since as of right now it only ever displays the input source anyway.