Things to ponder:

1) Should the stretch have been inserted "before the foo ended", i.e.,
with that face extending to the end of the range?

1a) Perhaps :extend of the face should control that?

2) To identify a range, we need an identity, so I used `eq' on the list
containing the width spec itself.  Other identities that are possible,
for instance:

(propertize "foo" 'display '(min-width 8.0 :the-identity))

But...  what if somebody then does:

(insert (propertize "foo" 'display '(min-width 5.0 :the-identity))
        (propertize "foo" 'display '(min-width 10.0 :the-identity)))

Using the `eq' of the list is awkward, but at least it's not ambiguous.
And we need an identity, because otherwise this won't be possible:

(insert (propertize "foo" 'display '(min-width 5.0))
        (propertize "bar" 'display '(min-width 5.0)))

3) Should this stuff nest?  We could have a stack of these, but meh.

4) Probably other things.

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   bloggy blog: http://lars.ingebrigtsen.no