A family of Microsoft word processing software products for creating web, email, and print documents.
None of them.
Even a brand new install of Word on a brand new computer has the same issue.
My point was that it's not limited to CTRL, SHIFT, ALT, or arrow-key navigations.
At this point I'm investigating non-Word options. The unpredictability of the problem makes it unreasonably difficult to troubleshoot, and the Microsoft help agents don't seem to understand that "it hasn't happened in the last 30 seconds" does not mean "the problem is fixed." I can continue to throw my valuable time into a black hole chasing a problem that MS doesn't seem to care about because it doesn't affect the average casual user, or I can find a solution that involves "not Microsoft."