Showing Output Frame Borders in VSE Preview Window


I am assembling a host of clips and images in Blender’s VSE. They are all of different sizes, meaning that I’ll be using the transform effect quite a lot to crop, scale and pan them.

My problem is that while I can zoom in and out of the preview window (and pan), I have not found any way of showing what part of the preview window will end up in the final output. Is there any way to turn on the same frame borders as one has in the modelling view?