m.KeyboardDialog = CreateObject("roSGNode", "KeyboardDialog")
m.KeyboardDialog.buttons = ["LOGIN","CANCEL"]
m.top.dialog = m.KeyboardDialog
This has to do with some funny behavior of roImageCanvas. It tops out at 1280x720, even if the UI is running FHD.
We just ran into this problem and were able to fix it. We looked at the KeyboardDialogExample and compared it to our code to see what was different. We weren't using a background image via the backgroundUri attribute, and as soon as we set that (using the image from the sample) it worked fine. :-/
So Roku folks, I would suggest that this is a bug - the KeyboardDialog's Keyboard node doesn't centre properly in the dialog when no backgroundUri is specified!