typical behavior for a fish about to give birth?

they will hover in a corner or secluded area and become nippier

My platies kind of disappear for a while. Someone on this site had one swimming around on top while delivering lots of fry.

Sometimes they will hide and you will not see them for a bit. Also they will hover above the gravel or if you have short grasses in a clump, they'll hover above that, Also they will breathe more heavily.

They can hover at the surface or near the substrate, but like stated, they do tend to retreat away before giving birth, probably a mechanism designed to ensure the fry aren't eaten right away.

The dark spot will get bigger and darker (these are the eyes of the babies). Not only will she get fatter but before giving birth her stomach will looked square, called squaring off as will the gravid spot. Mine like to hang out near the heater. Often will go off of their food, you might see them grab a piece of flake but then spit it out. Some though will be ravenous but most don't eat. Often give birth at dawn. Most swim in place, may dart forward a bit and then go back. Vent will open a little and ventral fins will be more spread open. For healthy fry feed the female a variety of foods - a little bit but often. The best advice I can give is to observe the fish over time, when you see behaviour which isn't typical she's likely in labour. If you want to save all of the fry I'd separate her when you see any of the above and add lots of plants. Move her to where you'll be raising the fry. When she's finished put her back so she doesn't eat the babies.

Mine swam up and down one side of the tank while giving birth