I've run into this use of も a few times and I'm not sure what to make of it. Here's the sentence I'm confused about: 余裕を見せていられるのも、いまの内だ。
The 「の」 is a nominalizing particle. It enables the verb phrase 「見せていられる」 to function as a noun.
The 「も」 is kind of like "also" but not quite in that it only hints at a possibility that there might be another thing that the speaker thinks applies besides 余裕を見せていられること.
「"Verb phrase in potential" + の + も + いまのうちだ。」 means:
"The situation where you could be (verb phrase) will not last long."
余裕を見せていられる = "(you could be) looking like you have leeway"