Heirarchical Category List: Non-empty sub-cats hidden

I have found this surprising hard to explain, so I will start with an illustration of a hypothetical category list (with post counts).

Cat1 (1)
-Cat1b (1)
Cat2 (0)
-Cat2b (1)
Cat3 (0)

So there are 2 posts, 1 in Cat1 and 1b, the other in Cat2b.
Naturally I want to hide Cat3 as it has no posts.

Unfortunately, using hide_empty=1 also hides Cat2, even though its sub-cat isn’t empty.

I could of course add the second post to the top-level cat (Cat2), but this is rather laborious, and may not be up to me anyway.

I could also use the exclude, but again that may be quite laborious and out of my control.

Can I recommend that list_cats be changed to check the sub-categories.