This seems to me a grey area. In compound adjectives, we normally
hyphenate when the lead word is an adverb not obviously so ("obvious"
usually meaning ending in '-ly'), as in the distinction between--
. a yellow window envelope (a window envelope of yellow paper, window
color not specified)
--and--
. a yellow-window envelope (a window envelope with a yellow-tinted
window, paper color unspecified).
The general use of the hyphen is to tell us that the hyphenated words
need to be interpreted as a unit to make the sense the writer intended
(as in the difference between a dancing girl and a dancing-girl). So on
the one hand, we can argue that as "ever" is always adverbial, no hyphen
is needed; but (I feel) there is a counter-argument that "ever-shorter"
is a unitary concept to a greater degree than the unhyphenated words
suggest.
But assuredly one would not use a hyphen in "Their supplies grew ever
shorter"; that is the same difference as in "It was a well-made cake" and
"The cake was well made", or "a rock-solid basis" and "a basis that is
rock solid".
--
Cordially,
Eric Walker