feat(transition | tui): add collapse transition animation

- add new `collapse.go` with collapse transition implementation
- implement edge-to-center collapsing animation with spring physics
- fix `expand.go` opposite method to return collapse instead of expand
- register collapse transition in transition factory

Author: tharropoulos

internal/tui/transitions/collapse.go

@@ -0,0 +1,131 @@
+package transitions
+
+import (
+	"math"
+	"strings"
+	"time"
+
+	tea "github.com/charmbracelet/bubbletea"
+	"github.com/charmbracelet/harmonica"
+	charmansi "github.com/charmbracelet/x/ansi"
+	"github.com/muesli/reflow/truncate"
+
+	"github.com/museslabs/kyma/internal/skip"
+)
+
+type collapse struct {
+	width     int
+	fps       int
+	spring    harmonica.Spring
+	progress  float64
+	vel       float64
+	animating bool
+	direction direction
+}
+
+func newCollapse(fps int) collapse {
+	const frequency = 7.0
+	const damping = 0.6
+
+	return collapse{
+		fps:    fps,
+		spring: harmonica.NewSpring(harmonica.FPS(fps), frequency, damping),
+	}
+}
+
+func (t collapse) Start(width, _ int, direction direction) Transition {
+	t.width = width
+	t.animating = true
+	t.progress = 0
+	t.vel = 0
+	t.direction = direction
+	return t
+}
+
+func (t collapse) Animating() bool {
+	return t.animating
+}
+
+func (t collapse) Update() (Transition, tea.Cmd) {
+	targetProgress := 1.0
+
+	t.progress, t.vel = t.spring.Update(t.progress, t.vel, targetProgress)
+
+	if t.progress >= 0.99 {
+		t.animating = false
+		t.progress = 1.0
+		return t, nil
+	}
+
+	return t, Animate(time.Duration(t.fps))
+}
+
+func (t collapse) View(prev, next string) string {
+	var s strings.Builder
+
+	// Calculate how much should collapse from edges toward center
+	collapseWidth := int(math.Round((1.0 - t.progress) * float64(t.width) / 2))
+	centerStart := t.width/2 - collapseWidth
+	centerEnd := t.width/2 + collapseWidth
+
+	prevLines := strings.Split(prev, "\n")
+	nextLines := strings.Split(next, "\n")
+
+	// Ensure slides are equal height
+	maxLines := max(len(nextLines), len(prevLines))
+
+	for i := range maxLines {
+		var prevLine, nextLine string
+
+		if i < len(prevLines) {
+			prevLine = prevLines[i]
+		}
+		if i < len(nextLines) {
+			nextLine = nextLines[i]
+		}
+
+		var line string
+		if collapseWidth <= 0 {
+			// Animation complete, show next content
+			line = truncate.String(nextLine, uint(t.width))
+		} else {
+			// Build the line with collapse effect from edges toward center
+			// Center portion: show prev content that's collapsing
+			var center string
+			if centerEnd > centerStart {
+				truncatedPrev := truncate.String(prevLine, uint(centerEnd))
+				center = skip.String(truncatedPrev, uint(centerStart))
+			}
+
+			// Left and right parts: show next content appearing from edges
+			leftNext := truncate.String(nextLine, uint(centerStart))
+
+			rightNext := ""
+			if centerEnd < t.width {
+				rightNext = charmansi.TruncateLeft(nextLine, centerEnd, "")
+			}
+
+			line = leftNext + center + rightNext
+			line = truncate.String(line, uint(t.width))
+		}
+
+		s.WriteString(line)
+		if i < maxLines-1 {
+			s.WriteString("\n")
+		}
+	}
+
+	return s.String()
+}
+
+func (t collapse) Name() string {
+	return "collapse"
+}
+
+func (t collapse) Opposite() Transition {
+	return newExpand(t.fps)
+}
+
+func (t collapse) Direction() direction {
+	return t.direction
+}

internal/tui/transitions/expand.go

@@ -123,7 +123,7 @@ func (t expand) Name() string {
 }
 
 func (t expand) Opposite() Transition {
-	return newExpand(t.fps)
+	return newCollapse(t.fps)
 }
 
 func (t expand) Direction() direction {

internal/tui/transitions/transition.go

@@ -45,6 +45,8 @@ func Get(name string, fps int) Transition {
 		return newSwipeRight(fps)
 	case "flip":
 		return newFlipRight(fps)
+	case "collapse":
+		return newCollapse(fps)
 	case "expand":
 		return newExpand(fps)
 	default: