The suite never selects a session, so initTerminal() does not run and both
`app.terminal` and `app.fitAddon` are null at rest. `_scheduleViewportSettle`
returns early on a falsy terminal, so the coalescing assertions could not
reach the behavior they claimed to cover -- the test errored on
`Cannot read properties of null` rather than measuring anything.
Installs the minimum surface the settle callback touches and restores it
afterwards, so the coalescing path executes for real.
Adds a behavioral counterpart driven through the PUBLIC entry point
(`onKeyboardShow`) instead of the internal scheduler: three viewport steps
in quick succession must produce exactly ONE refit. On master that returns
3 (each show arms its own uncoalesced 150ms timeout), so this fails by
COUNT rather than by a missing method -- which is the failure mode that
actually demonstrates the bug.
Verified: `expected 3 to be 1` on unmodified master; passes here. The
remaining 8 failures in this file are pre-existing on master and unrelated
(same null-initialization limitation of the headless harness).
lior