fix bug where help might display in corner

This fixed a bug that might cause the help to display in the top left corner of the screen.

As part of a 'hack' to handle backspace in the completion list, we display the popup off-screen, but allow the current 'completion context' to remain active. This ensures that the current set of completions can be displayed if you mistype a token and later hit backspace.

We now double-check that the completion list is actually on-screen before displaying help.