Midwest, USA. Over a hundred year old house.

This is a typical sliding glass door, so I doubt it's original. I've seen them before -- Outermost sliding screen. A non-sliding half that stays there. And then sliding door that weighs a ton.

This is about the non-sliding door slide, the one stuck in place.

I did take pics and can upload them but there's not much to add. I noticed on the NON-sliding glass door side that it's that big window pane. Under that, there's another strip of wood that's maybe a 1x1 or 2x4. I think that non-moving side of glass sits on that, probably (but that's why I'm asking here too). And then I think it's the trim and opening in the wall.

The problem -- That strip of wood under the non-sliding glass door looks pretty decayed. It's dry though. It's more like the wood dried but weathered so much you can just push through it. It's more on the middle/inner side, closer to the moving glass door side. I noticed it doesn't have paint. The paint on the sliding door trim is peeling off.…