io.github.libtmux.Window.Window.applyLayout

The string is checked here rather than by tmux, because tmux 3.3a does not survive being handed one it cannot parse: it ends the server and every session on the socket. Every other supported release answers invalid layout. The classic form carries tmux's own checksum, so a wrong one is detectable without asking; since 3.8 tmux may instead hand back JSON, which carries no checksum, so that shape is only trusted from a server new enough to have written it.

"Exact" holds in full only for a JSON layout, since that is the one form carrying each pane's id. A classic string — every release before 3.8, or an older reader against a newer server — restores the geometry but names no pane, so which process lands in which cell can differ from where it started; whether it does depends on whether the pane list still happens to match the order the layout was saved in. Confirmed by hand on 3.2a and 3.7c.