Zero is always in alt-screen mode (useAltScreen() is unconditionally
true), and transcriptView's `if m.altScreen && m.height > 0` gate is what
pads the rendered frame out to fill the full terminal height
(scrollableTranscriptItemsView pads bodyWindow to window.height). If
m.height is ever 0 - Bubble Tea documents an initial WindowSizeMsg as
delivered automatically on program start, but that's the terminal
proactively pushing a size, and nothing else ever asks again if that
push is missed (a slow/unusual terminal, a multiplexer, a startup race)
- Zero falls back to the unpadded, non-fullscreen render path for the
rest of the session: content only occupies as many rows as it needs,
leaving the remaining terminal height as plain unfilled background
instead of the alt-screen's painted panel color.
Fixed by explicitly firing tea.RequestWindowSize in Init(), the same
defensive pattern already used for tea.RequestBackgroundColor right below
it - Zero doesn't rely solely on an unprompted push for either.