Onscreen Keyboard cannot be used to input password

marmuta says in duplicate bug 852032:
"I've seen this too. This is just a guess, but the reason may be, that there is no real input focus on the password entry field. Onboard uses XTestFakeKeyEvent to send keystrokes. If there is no focussed window the keystrokes don't go anywhere."

An observation:
The focus follows the mouse, i.e. with the mouse over the password entry you can use the up/down keys. When over the onscreen keyboard up/down doesn't work. This is probably due to there not being a window manager running.

That would explain it. With focus-follows-mouse Onboard sends keystrokes at best to itself.

I wonder if a simple XSetInputFocus() on the password entry would help.
"It causes keyboard input to go to the selected window regardless of the position of the pointer. When the root window is selected, keyboard events are distributed normally according to the position of the pointer (this is the default situation)." [1]

I would try it with unity-greeter, but I don't know how to call xlib from vala.

FWIW, I had a quick look at the sources of GDM and KDM. GDM actually starts either metacity or compiz for a click-to-focus policy.
KDM seems to do without a window manager. It appears to hold a keyboard grab and manually creates fake focus events into the Qt library as needed.

[1] "Changing Keyboard Focus", http://www.sbin.org/doc/Xlib/chapt_16.html

Fixed in 0.0.9

the new version is in Oneiric