My apologies if this trick is already well known and well documented, or if there is a simpler, better way to do it. But I figured it out on my own last night and just had to tell someone about it.

I had a 45" long closet wall in which I needed to put a sliding door, and above that a window. (It was actually another sliding door for an overhead bin above the closet, but the customization I could do to a window looked better than the options for doors.)
When you stack things (doors, windows) in a wall, you have to work from bottom to top, that is, the lowest element has to be placed first. If I placed the door, then the window wouldn't place because the door was occupying space that TFP needed to place the window at its default height, and you can't elevate an element until after it is placed. The wall was not long enough to place the two elements side by side.
After some head scratching, it occurred to me that all I had to do was elevate the 80" tall door to -70", and then I could place the window, elevate it to the desired height of 110" (the wall is 120 inches high), then go back and re-elevate the door back to its default height of 0". Note that the elevation height of windows is shown as the height of the
top of the window, and for doors it is the
bottom of the door.
I guess that for someone who works with the program all the time and knows all the ins and outs of it (yes, I'm talking about you, Allan!

), this wouldn't be such a big deal but for me it was a true epiphany.
tanstaafl.