Android or iOS? They handle this differently. On iOS generally it keeps the viewport the same size and shifts the text field in to the visible part of the viewport and everything works OK. On Android though currently the keyboard changes the viewport size, which kicks off a fullscreen scaling resize, which then changes the textbox size. Then you get in to all sorts of awkward situations where you can't see the input, or it scrolls away and therefore hides the field so the keyboard closes, etc. It's made harder by the fact browsers do not provide any way to detect if the keyboard is currently showing, and just to throw another spanner in the works, Android is currently changing the way they handle this, so if we fix it now it could be broken again shortly. All in all, a big headache.
Generally we don't fix bugs unless there's a report filed for them - can you link me to any relevant bug reports so I can take another look? It's probably going to involve some kind of ugly hack though...