The delete-confirmation dialog mounted as a sibling below the picker's
bottom border briefly grew the picker past the terminal height. The TUI's
append-only renderer committed the picker's top rows (header + first
sessions) into native scrollback to fit the dialog within the viewport.
When the dialog closed and the picker re-rendered shorter, `windowTop`
stayed pinned at `#committedRows`, leaving the picker stranded below the
committed prefix — the user saw the header scrolled off the top.
SessionList now exposes `setExternalReserveRows`; SessionSelectorComponent
reserves the dialog's worst-case height while the dialog is mounted so
the picker's total rendered output stays within the terminal viewport
and the TUI never commits its rows.
Fixes#3283
roboomp
ecdff42513