One thing that might be worth a shot is some of that liquid masking. One I have heard good things about is Jasco Mask and Seal. We used to just spray it over the whole window and the product would sink into the wood but not the glass so that after you are done painting you just peel it off the glass. What you can do is just use a brush and it's okay if you get some on the wood because after you are done painting you will just score the line between the wood and glass and just peel the liquid masking off. Just don't leave it on too long before removing it....follow the manufacturers recs concerning that.

Try using a 2" angled trim brush. I have found I can do a better job with a larger brush and in fact I use a 3" brush on most of my trim and cutting in work. Maybe it's just what I'm used to but I think it really works. I do have a theory on it but don't care to go into it now.

Don't start with a loaded brush next to the glass, start your stroke maybe 1/4" away and slide the brush to the glass as you stroke.

My twin "bad" brother came up with this - when you're 16 feet down and looking up at a window maybe you just can't see that 1/16" next to the glass that didn't get a fresh coat of white over the old white.