Try and save a project from being in a full window
It's already more information, but it's still unclear what is the unwanted result that occurs when following these steps. Does Blender crash, freeze, UI artifacts appear, save window doesn't open, lag?
EDIT: I checked the screenshot again and it looks like the file editor opens, but only its header is visible, the rest is from the previous 3d editor. Is this the bug your are reporting?