Holding D in develop across the edge of the loaded window panicked with
"RefCell already borrowed" at the first step that had to move it. The
`*ctl.offset.borrow()` written inside the `if let` condition is a
temporary that lives to the end of the `if let` block (edition 2021),
and the block calls `load_window`, which borrows the offset mutably.
The unit tests drive the placement arithmetic, not the RefCells, so
they could not see it; stepping 400 frames in the app did.
The offset is copied out before the test. `follow_open` gets the same
treatment for `image_ids`: the lookup's result is bound first, so no
borrow is held while it writes properties back to the window.
dtourolle
19dd3257e3