It turns out — and thanks to Jon Gotow of St. Clair Software, maker of the excellent Default Folder X, for the answer to this — that there’s a bug in Yosemite that causes a sheet to grow taller by 22 pixels every time you use it.
This is such a weird bug, but so easy — and fun! — to reproduce. Just hit
Cmd + S in this window, then hit
Esc. Rinse and repeat until your Safari window is moving way off-screen to fit the enormous Save dialog.
Reminds me of that amazing Finder bug that Cabel Sasser found years ago.