Thanks for that info. That post was from Oct. 2017. There have been updates to the OS since then.
My problem just started after the last update. I don't get to "hold" Recent Apps button, all I do is barely touch it and the "split screen" pops up so fast that I can't access the "close all apps" button it's covering up. I finally discovered that hitting the Return or Back button on the bottom right side of the phone will close the Split Screen window so I can access the "close apps" button.
This appears to be a bug in the updated program. They took the "flip to silence" feature out with this update too, something I really miss. Maybe they will change it again with the next update.