Yes, this should be mutable (and you're right that relative does for a better workflow), however the relative URL is taken relative to the hg root repo where .hgsub lives .hgsub - not where you click (which comes from .hg/hgrc deafult in the paths section.
Here is a pretty normal subrepo layout:
on server http://hgserver/disp/main http://hgserver/disp/common # the "common" repo http://hgserver/disp/main # the main repo http://hgserver/disp/main/.hgsub
Then, after cloning, everything works, and the same hgsub works fine on the server as well.
There are many questions about the stack overflow, where people look at the best layouts for the relative settings of the subrepost, and although I did not try to switch from one to the other, I think that if you create a style “next to” the sub-repo with "../sibling" I show above, this will work fine.
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