Because there is no class ExtPage (yet) and because the dialog has gridded widgets, an error message said the label frames had to be gridded rather than packed. I don't know why the same is not true within the frame classes. Anyway, I just guessed at what the arguments should be after the change to grid. I stuck a blank label between the two label frames so they were not jammed together. There is probably a better way. This page definitely needs a followup, tested on all 3 OSes (see below).

Maybe all the pages would be better gridded, but by someone who understands the arguments. Maybe we should look at the various themes available on each OS. Or create our own. And definitely reconsider the look and feel. As far as I know, the reliefs are as they were 20 years ago. I'd like to try giving the labelled frame a different background from the page frames. But this is a different discussion, to be done after we have the right widgets present.

The only paned window I am familiar with is turtledemo. At least on Windows, the divider is quite different from anything on this dialog.

(Edited from here.)
You must be on Linux. When I select the Extensions tab, zzdummy is selected and its detail present. (Even though current extension is set to None.) I cannot unselect it. So I see no height change. On the other hand, I see space under the helplist box.

The checkboxes are buggy. Selecting them displays a dotted rectangle to the right, which can be clicked on to uncheck and check the boxes. I see blank space below the helplist box, indicating that this page is not forcing the dialog height.