ἡττᾶσαι (post-Classical)--why doesn't the -σ- drop out?

Greek Learning Greek
1

From Evagrius (4th cent. AD), De oratione, 41 (in the critical edition by Paul Géhin):
Ὅρα, εἰ ἀληθῶς Θεῷ παρέστηκας ἐν τῇ προσευχῇ σου ἢ ἐπαίνῳ ἀνθρωπίνῳ ἡττᾶσαι καὶ τούτον θηρᾶν ἐπείγῃ, ὥσπερ ἐπικαλύμματι κεχρημένος τῇ παραστάσει τῆς προσευχῆς.
TLG gives “pres subj/ind mid-pass 2nd sg [byz/demotic].”
The editor lists this verb in infinitive as ἡττᾶν, so the Classical pres subj/ind mid-pass 2nd sg would be ἡττᾷ. Whence, then, ἡττᾶσαι? Is it regular in later Greek that the -σ- does not drop out? And if so, is there a grammar book that gives all later forms? [My apologies if this should have been posted rather on the “Medieval Greek” forum, which I have just noticed.]

2

this is regular in NT Greek for contracted verbs, see, Blass, 21, 8

3

If the sigma is part of the stem, or part of the ending, such as -σαι for the aorist infinitive active, then the “sigma drop” rule doesn’t apply.

4

Thanks, this is very helpful!