στέργημα γὰρ δοκοῦσα [Deianeira] προσβαλεῖν σέθεν [Heracles]
ἀπήμπλαχ’, ὡς προσεῖδε τοὺς ἔνδον γάμους.
Deianeira was declared dead in lines 874-5.
Jebb on δοκοῦσα in S.Tr.1139 refers to his note on S.Ant.166. There Ant.169 and 1192 and OT.835 are cited as examples of present participles that are equivalent to subordinate clauses with an imperfect indicative.
Jebb also directs the reader to his note on S.OC.1565 f. He discusses the problematic nature of this alleged instance in comparison with passages where the context removes any ambiguity: OC.1587, OT.835 (again), X.An.5.8.1, and D.20.119.
In his note to S.OT.835, D.19.129 is cited as a comparandum.