fix(tui): explicitly request the initial window size at startup

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.

internal/tui/init_window_size_test.go

@@ -0,0 +1,59 @@
+package tui
+
+import (
+	"context"
+	"reflect"
+	"testing"
+
+	tea "charm.land/bubbletea/v2"
+)
+
+// TestInitRequestsWindowSize: Bubble Tea documents an initial WindowSizeMsg as
+// delivered automatically on program start, but that's the terminal
+// proactively pushing a size — if it's ever missed (a slow/unusual terminal,
+// a multiplexer, a startup race), nothing else asks again, m.height stays 0
+// forever, and transcriptView's `if m.altScreen && m.height > 0` gate falls
+// back to the unpadded, non-fullscreen render path for the whole session (the
+// alt-screen viewport never gets filled below the actual content). Init must
+// explicitly request it too, the same way it already explicitly requests the
+// background color rather than relying solely on an unprompted push.
+func TestInitRequestsWindowSize(t *testing.T) {
+	m := newModel(context.Background(), Options{ModelName: "gpt-4"})
+	cmd := m.Init()
+	if cmd == nil {
+		t.Fatal("Init() must return a non-nil command")
+	}
+
+	want := reflect.ValueOf(tea.RequestWindowSize).Pointer()
+	if !batchContainsCmd(t, cmd, want) {
+		t.Error("Init() must request the window size (tea.RequestWindowSize), not rely solely on the terminal's unprompted initial WindowSizeMsg")
+	}
+}
+
+// batchContainsCmd reports whether cmd (Init()'s single top-level
+// tea.Batch(...)) contains a sub-command matching want, compared by pointer
+// (the same technique TestThemeAutoReProbesBackground uses for
+// tea.RequestBackgroundColor — reliable for named top-level functions like
+// tea.RequestWindowSize, not closures). Invokes cmd itself once to unpack the
+// tea.BatchMsg, but only pointer-compares the sub-commands — never calls
+// them, since Init()'s batch also carries real closures (e.g. provider model
+// discovery) that make live network calls and must not fire in a unit test.
+func batchContainsCmd(t *testing.T, cmd tea.Cmd, want uintptr) bool {
+	t.Helper()
+	if cmd == nil {
+		return false
+	}
+	if reflect.ValueOf(cmd).Pointer() == want {
+		return true
+	}
+	batch, ok := cmd().(tea.BatchMsg)
+	if !ok {
+		return false
+	}
+	for _, sub := range batch {
+		if sub != nil && reflect.ValueOf(sub).Pointer() == want {
+			return true
+		}
+	}
+	return false
+}

internal/tui/model.go

@@ -790,6 +790,18 @@ func composerBlinkCmd() tea.Cmd {
 
 func (m model) Init() tea.Cmd {
 	cmds := []tea.Cmd{textinput.Blink, composerBlinkCmd()}
+	// Bubble Tea documents an initial WindowSizeMsg as delivered automatically
+	// on program start, so m.height/m.width are normally set before the first
+	// render. But that's the terminal proactively pushing a size — if it's
+	// ever missed (a slow/unusual terminal, a multiplexer, a startup race),
+	// nothing else ever asks again: m.height stays its zero value forever,
+	// `if m.altScreen && m.height > 0` (transcriptView) falls back to the
+	// unpadded, non-fullscreen render path for the rest of the session, and
+	// the alt-screen viewport never gets filled below the actual content.
+	// Explicitly requesting it here means Zero doesn't depend solely on the
+	// terminal's unprompted push — mirrors the RequestBackgroundColor request
+	// below for the same reason.
+	cmds = append(cmds, tea.RequestWindowSize)
 	// In auto mode, ask the terminal for its background color; the reply arrives
 	// as tea.BackgroundColorMsg and selects light vs dark (see updateModel).
 	if m.themeMode == themeAuto {