fix scroll bug when returning to the list view of commits

Author: skunkworker

main.go

@@ -367,6 +367,19 @@ func (m *model) clampCursor() {
 	m.adjustScroll()
 }
 
+// scroll applies a mouse-wheel step (delta of -1 up / +1 down) to whichever
+// scrollable view is active; other states ignore the wheel.
+func (m *model) scroll(delta int) {
+	switch m.state {
+	case stateList:
+		m.cursor += delta
+		m.clampCursor()
+	case stateDiff:
+		m.diffTop += delta * 3 // 3 lines per wheel notch, like a pager
+		m.clampDiff()
+	}
+}
+
 func (m *model) visibleRows() int {
 	return max(1, m.height-5) // minus header (2) + footer (3)
 }
@@ -450,6 +463,16 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 			return m, spinnerTickCmd()
 		}
 		return m, nil
+	case tea.MouseMsg:
+		// Handle the wheel ourselves so it scrolls the active view rather than
+		// the terminal translating it into arrow-key bursts that leak between views.
+		switch msg.Button {
+		case tea.MouseButtonWheelUp:
+			m.scroll(-1)
+		case tea.MouseButtonWheelDown:
+			m.scroll(1)
+		}
+		return m, nil
 	case tea.KeyMsg:
 		switch m.state {
 		case stateList:
@@ -1201,7 +1224,7 @@ func main() {
 		fmt.Fprintln(os.Stderr, "git_pruner:", err)
 		os.Exit(1)
 	}
-	p := tea.NewProgram(m, tea.WithAltScreen())
+	p := tea.NewProgram(m, tea.WithAltScreen(), tea.WithMouseCellMotion())
 	if _, err := p.Run(); err != nil {
 		fmt.Fprintln(os.Stderr, "git_pruner:", err)
 		os.Exit(1)

main_test.go

@@ -507,6 +507,44 @@ func TestRemoteMergedIndicator(t *testing.T) {
 	}
 }
 
+// Scrolling the diff view with the wheel must not disturb the list's cursor or
+// scroll position (regression: over-scrolling the diff jumped the list to the end).
+func TestWheelScrollDoesNotLeakBetweenViews(t *testing.T) {
+	repo := setupRepo(t)
+	chdir(t, repo)
+	m, err := initialModel()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Park the list cursor at a non-zero row.
+	m.cursor = 2
+	m.clampCursor()
+	savedCursor, savedTop := m.cursor, m.top
+
+	// Enter the diff with scrollable content and scroll it hard past the bottom.
+	m.state = stateDiff
+	m.diffLines = make([]string, 200)
+	m.diffTop = 0
+	for range 500 {
+		m.scroll(1)
+	}
+	if m.diffTop == 0 {
+		t.Fatal("diff should have scrolled down")
+	}
+	if m.cursor != savedCursor || m.top != savedTop {
+		t.Fatalf("diff scroll leaked into list: cursor %d->%d, top %d->%d",
+			savedCursor, m.cursor, savedTop, m.top)
+	}
+
+	// Back in the list, the wheel drives the cursor (routed via Update/MouseMsg).
+	m.state = stateList
+	nm, _ := m.Update(tea.MouseMsg{Button: tea.MouseButtonWheelDown, Action: tea.MouseActionPress})
+	if got := nm.(model).cursor; got != savedCursor+1 {
+		t.Fatalf("list wheel should move cursor to %d, got %d", savedCursor+1, got)
+	}
+}
+
 // The version string is well-formed even when VCS build info is absent.
 func TestVersionString(t *testing.T) {
 	s := versionString()