Just tried this and it worlks a treat. Don't use masking take on the glass, use vaseline on the glass to act as a paint barrier. Work in well with a finger, smearing the vaseline a good half inch from the window frame. Don't get the vaseline on the woodwork itself. Now you can paint all the tricky inner edges. When the paint is dry wash the glass thoroughly with warm water and detergent using a soft scourer. Result - neatly painted windows with shining glass!

If you get paint splashes or drops on glass an old fashioned razor blade is great a removing it and it doesn't scratch the glass

Put cellotape around the inside of the window frame, so that when the paint has dried you can just peal it away, leaving nice clean windows :) :)

use painters tape

EASY. JUST PASTE A PAPER ONTO THE GLASS AND PAINT THE FRAME.

Masking tape!