Clicking with the mouse is actually a decent way of moving diagonally on a laptop without a numpad. But sometimes, due to the "pseudo-3D" effect of the tiled layout, it's easy to misclick on the tile just above or below the one you actually want. This is related with the issue just reported by Grey with monsters ( http://www.adom.de/forums/project.php?issueid=2146 ) but it can happen also with doors and corridors.
I think a good solution would be to subtly highlight the tile the cursor is over at each given moment. Something like what's currently implemented in look mode, but maybe with a more subtle highlight (to actually tell it from look mode).