I usually either accept or work around Word's grammar suggestions (I hate having red/blue lines in my documents) but this particular suggestion has me stumped. The sentence is "Striding forward with purpose, she bit her lip and suppressed a shiver." Word insists that this is a comma splice, but "Striding forward with purpose." would be a sentence fragment due to the lack of subject, right? Am I being an idiot?
How strange: I tried it in Word, and it found nothing to grumble about.
It's not a comma splice. The non-finite participial clause "Striding forward with purpose" is a supplementary adjunct. Most non-finite clauses have no overt subject, and yours is no exception, but we understand them as having subjects. In your example, the subject is retrievable by looking at the subject of the matrix clause. It then becomes clear that the subject is "she"; we understand that it was "she" who was striding forward with purpose.
Supplements like the one in your example are usually set of with a comma (or commas) so there’s nothing at all wrong with your sentence.