I am struggling with "Mother of Millions" in the workbook as well. The sources list disappears as soon as I click the zoom tool. The panel is there but it's empty. I have a hi-res 24" monitor but there's no way I can edit the stack accurately without zooming. This lesson involves cloning out halos, and it's not possible to zoom in to see the halos and go back to sources to clone from, so it's not possible to complete this lesson.
I can replicate this easily. New focus merge, select any files, when the source list populates, click on any tool, and "poof". The source panel is empty. It doesn't matter how many images in the stack.
I searched the bug forum but did not see this mentioned. Is this a known issue being worked on?
MacBook Air, 1.7GHz Core i7, Intel HD Graphics 5000 1536MB, 8GB ram, High Sierra 10.13.4, Affinity Photo 1.6.7