feat: session scrollback with mouse wheel scroll

cmd/dj/main.go

@@ -48,7 +48,7 @@ func runApp(cmd *cobra.Command, args []string) error {
 		tui.WithInteractiveCommand(cfg.Interactive.Command, cfg.Interactive.Args...),
 	)
 
-	program := tea.NewProgram(app, tea.WithAltScreen())
+	program := tea.NewProgram(app, tea.WithAltScreen(), tea.WithMouseCellMotion())
 	finalModel, err := program.Run()
 
 	if finalApp, ok := finalModel.(tui.AppModel); ok {

docs/plans/2026-03-18-session-scrollback-design.md

@@ -0,0 +1,60 @@
+# Session Scrollback Design
+
+## Problem
+
+When a codex CLI session produces output that scrolls past the visible area, the user cannot scroll up to review earlier output. The vt emulator stores a scrollback buffer, but the TUI does not expose it.
+
+## Approach
+
+Add scroll state directly to `PTYSession`. The emulator already maintains a scrollback buffer via `Scrollback()`. When the user scrolls up with the mouse wheel, the `Render()` method builds a custom viewport from scrollback lines + visible screen lines instead of calling `emulator.Render()`.
+
+## Design
+
+### Scroll state on PTYSession
+
+Add to `PTYSession`:
+- `scrollOffset int` — 0 means at bottom (live), positive means lines scrolled up
+- `ScrollUp(lines int)` — increase offset, clamped to max scrollback
+- `ScrollDown(lines int)` — decrease offset, clamped to 0
+- `ScrollToBottom()` — reset offset to 0
+- `IsScrolledUp() bool` — returns `scrollOffset > 0`
+- `ScrollOffset() int` — returns current offset
+
+### Custom viewport rendering
+
+When `scrollOffset > 0`, `Render()` builds output by:
+1. Collecting all scrollback lines via `Scrollback().Lines()`
+2. Collecting visible screen lines via `CellAt(x, y)` for each row
+3. Concatenating into one logical buffer (scrollback on top, screen on bottom)
+4. Slicing a window of `emulator.Height()` lines, offset from the bottom by `scrollOffset`
+5. Converting cells to styled strings for display
+
+When `scrollOffset == 0`, `Render()` calls `emulator.Render()` as before.
+
+### Mouse input
+
+- Enable mouse mode with `tea.WithMouseCellMotion()` in program options
+- Handle `tea.MouseMsg` in `Update()`:
+  - Scroll wheel up → `ScrollUp` on active PTY session
+  - Scroll wheel down → `ScrollDown` on active PTY session
+- Do not forward scroll wheel events to the PTY process
+- Non-scroll mouse events are not forwarded (PTY apps that need mouse input are out of scope)
+
+### Auto-scroll behavior
+
+When new output arrives while scrolled up, the view stays in place. The user must scroll down manually or the offset resets on keyboard input to the PTY.
+
+### Scroll indicator
+
+When `IsScrolledUp()` is true, render a bottom-line indicator in the session pane:
+- Format: `↓ N lines below`
+- Styled with a distinct background so it overlays the content visibly
+- Disappears when scroll offset returns to 0
+
+## Files changed
+
+- `internal/tui/pty_session.go` — scroll state, modified `Render()`
+- `internal/tui/app.go` — mouse message handling in `Update()`
+- `internal/tui/app_pty.go` — scroll dispatch for active session
+- `internal/tui/app_view.go` — scroll indicator overlay in `renderPTYContent()`
+- `cmd/dj/main.go` — add `tea.WithMouseCellMotion()` to program options