fix(tui): address slack-handling review findings

Apply review feedback on the bottom-slack work:

- Rename onAnimationTick to handleAnimationTick to match the
  surrounding handle* convention (handleMouseClick, handleKeyPress,
  etc.).
- Apply maxBottomSlack() to the public AdjustBottomSlack API so
  external callers cannot bypass the cap that protects the viewport
  from going mostly empty.
- Strengthen TestBottomSlackAnimationSubscribesWhileDecaying so it
  also asserts that Update returns a non-nil tick command — without
  it, the subscription would be marked active but no tick would ever
  be scheduled.
- Add TestBottomSlackDecayPausesWhenUserScrollsAway covering the
  case where the user scrolls away mid-decay.
- Add TestAdjustBottomSlackRespectsCapAndFloor covering the new cap
  and the existing floor on AdjustBottomSlack.

Assisted-By: docker-agent

Author: dgageot

pkg/tui/components/messages/messages.go

@@ -277,7 +277,7 @@ func (m *model) Update(msg tea.Msg) (layout.Model, tea.Cmd) {
 	// applied their fade state, and before tui.go's HasActive() check so the
 	// subscription is registered when the next tick is scheduled.
 	if _, ok := msg.(animation.TickMsg); ok {
-		cmds = append(cmds, m.onAnimationTick())
+		cmds = append(cmds, m.handleAnimationTick())
 	}
 
 	return m, tea.Batch(cmds...)
@@ -526,9 +526,9 @@ func (m *model) View() string {
 	}
 
 	m.updateScrollState()
-	// Release the slack subscription once it's no longer needed. The reverse
-	// (starting it) is only done from Update via onAnimationTick, where the
-	// returned tea.Cmd can be propagated to actually schedule the next tick.
+	// Release the slack subscription once it's no longer needed. Starting it
+	// is only done from Update via handleAnimationTick, where the returned
+	// tea.Cmd can be propagated to actually schedule the next tick.
 	if m.bottomSlack == 0 {
 		m.slackAnimationSub.Stop()
 	}
@@ -614,11 +614,11 @@ func (m *model) maxBottomSlack() int {
 	return max(1, min(5, m.height/3))
 }
 
-// onAnimationTick refreshes scroll state, decays any leftover slack by one
-// line, and keeps the slack subscription alive while slack > 0 so further
-// ticks fire even after fade animations finish. Returns the command to
-// schedule the next tick when the subscription transitions to active.
-func (m *model) onAnimationTick() tea.Cmd {
+// handleAnimationTick refreshes scroll state, decays any leftover slack by
+// one line, and keeps the slack subscription alive while slack > 0 so
+// further ticks fire even after fade animations finish. Returns the command
+// to schedule the next tick when the subscription transitions to active.
+func (m *model) handleAnimationTick() tea.Cmd {
 	m.updateScrollState()
 	if !m.userHasScrolled && m.bottomSlack > 0 {
 		m.bottomSlack--
@@ -1584,7 +1584,7 @@ func (m *model) AdjustBottomSlack(delta int) {
 	if delta == 0 {
 		return
 	}
-	m.bottomSlack = max(0, m.bottomSlack+delta)
+	m.bottomSlack = max(0, min(m.bottomSlack+delta, m.maxBottomSlack()))
 }
 
 // contentWidth returns the width available for content.

pkg/tui/components/messages/slack_test.go

@@ -111,11 +111,14 @@ func TestBottomSlackAnimationSubscribesWhileDecaying(t *testing.T) {
 		"no slack means no animation subscription")
 
 	// Slack > 0: a tick should subscribe so further ticks keep firing
-	// even after fade animations finish.
+	// even after fade animations finish, and Update must return the
+	// scheduling command so Bubbletea actually delivers the next tick.
 	m.bottomSlack = 2
-	m.Update(animation.TickMsg{Frame: 1})
+	_, cmd := m.Update(animation.TickMsg{Frame: 1})
 	assert.True(t, m.slackAnimationSub.IsActive(),
 		"subscription should be active while slack > 0")
+	assert.NotNil(t, cmd,
+		"Update must return a tick command so the next tick is scheduled")
 
 	// Once slack hits zero, the subscription must release the global tick.
 	m.Update(animation.TickMsg{Frame: 2})
@@ -198,3 +201,55 @@ func TestBottomSlackIsZeroWhenUserHasScrolledAway(t *testing.T) {
 	assert.Equal(t, 0, m.bottomSlack,
 		"slack must remain zero when the user scrolled away from the bottom")
 }
+
+func TestBottomSlackDecayPausesWhenUserScrollsAway(t *testing.T) {
+	t.Parallel()
+
+	sessionState := &service.SessionState{}
+	m := NewScrollableView(80, 24, sessionState).(*model)
+	m.SetSize(80, 24)
+
+	addShrinkingView(m, 10)
+	m.View()
+
+	// Pretend a previous shrinkage left some slack while auto-following.
+	m.bottomSlack = 3
+
+	// First tick decays one line as expected.
+	m.Update(animation.TickMsg{Frame: 1})
+	require.Equal(t, 2, m.bottomSlack)
+
+	// User scrolls away mid-decay. updateScrollState resets slack to zero
+	// for the userHasScrolled path; the next tick should leave it there
+	// and not produce a negative value.
+	m.userHasScrolled = true
+	m.Update(animation.TickMsg{Frame: 2})
+	assert.Equal(t, 0, m.bottomSlack,
+		"slack must drop to zero (not be decayed below it) once the user scrolls away")
+}
+
+func TestAdjustBottomSlackRespectsCapAndFloor(t *testing.T) {
+	t.Parallel()
+
+	sessionState := &service.SessionState{}
+	m := NewScrollableView(80, 24, sessionState).(*model)
+	m.SetSize(80, 24)
+
+	maxSlack := m.maxBottomSlack()
+
+	// Adding more than the cap must clamp to the cap.
+	m.AdjustBottomSlack(100)
+	assert.Equal(t, maxSlack, m.bottomSlack,
+		"AdjustBottomSlack must clamp positive deltas to maxBottomSlack")
+
+	// Subtracting below zero must clamp to zero.
+	m.AdjustBottomSlack(-100)
+	assert.Equal(t, 0, m.bottomSlack,
+		"AdjustBottomSlack must clamp negative deltas at zero")
+
+	// Zero delta is a no-op.
+	m.bottomSlack = 2
+	m.AdjustBottomSlack(0)
+	assert.Equal(t, 2, m.bottomSlack,
+		"AdjustBottomSlack(0) must leave slack unchanged")
+}