From 04069e19682c3c20d19ad7492dc53fb4b8f6879d Mon Sep 17 00:00:00 2001
From: Alexander Alber <90604107+alexanderalber@users.noreply.github.com>
Date: Sun, 11 Oct 2026 09:03:01 +0200
Subject: [PATCH] Morph: text boxes without a partner fade in place instead of
 moving (#119)

The last matching pass paired any two shapes of one kind and preset
geometry, so a text box leaving the slide was paired with an unrelated
one arriving and flew across to it. That pass now only pairs shapes
without text: a text box needs its name or its text on the other slide
to morph, otherwise it fades out (or in) where it is, as in PowerPoint.

generated with Claude Code
---
 crates/anim/src/morph.rs |  7 +++++--
 crates/anim/src/tests.rs | 20 ++++++++++++++++++++
 docs/animation-parity.md |  3 ++-
 3 files changed, 27 insertions(+), 3 deletions(-)

diff --git a/crates/anim/src/morph.rs b/crates/anim/src/morph.rs
index b5808c4f..1d4ab8f3 100644
--- a/crates/anim/src/morph.rs
+++ b/crates/anim/src/morph.rs
@@ -12,7 +12,9 @@ fn text_of(s: &Shape) -> String {
 
 /// Pairs of (shape on `a`, shape on `b`) that Morph animates from one to the other: first by
 /// `!!name` (forced match), then by exact name, then by the same kind and text, then by the same
-/// kind and preset geometry. Each shape is used once; top-level shapes only (groups move whole).
+/// kind and preset geometry if neither carries text. A text box with neither a name nor its text on
+/// the other slide stays unpaired and fades in place, as in PowerPoint (#119). Each shape is used
+/// once; top-level shapes only (groups move whole).
 pub fn morph_pairs(a: &Slide, b: &Slide) -> Vec<(ShapeId, ShapeId)> {
     let mut pairs: Vec<(ShapeId, ShapeId)> = Vec::new();
     let mut used_a = vec![false; a.shapes.len()];
@@ -31,7 +33,8 @@ pub fn morph_pairs(a: &Slide, b: &Slide) -> Vec<(ShapeId, ShapeId)> {
             x.kind_name() == y.kind_name()
                 && x.geom.preset_name().is_some()
                 && x.geom.preset_name() == y.geom.preset_name()
-                && text_of(x).trim().is_empty() == text_of(y).trim().is_empty()
+                && text_of(x).trim().is_empty()
+                && text_of(y).trim().is_empty()
         },
     ];
     for pass in passes {
diff --git a/crates/anim/src/tests.rs b/crates/anim/src/tests.rs
index 1fd82d78..de9c40ab 100644
--- a/crates/anim/src/tests.rs
+++ b/crates/anim/src/tests.rs
@@ -529,6 +529,26 @@ fn morph_pairs_by_name_text_and_geometry() {
     assert!(morph_pairs(&Slide::default(), &b).is_empty());
 }
 
+#[test]
+fn morph_pairs_leaves_unrelated_text_boxes_alone() {
+    // A text box that leaves and an unrelated one that arrives fade in place (#119).
+    let p = deckcraft_model::defaults::new_presentation(None);
+    let mut a = Slide::default();
+    let mut b = Slide::default();
+    let tb = |id, name, x, text| Shape { xfrm: Some(Xfrm::new(x, 100.0, 200.0, 100.0)), text: Some(TextBody::from_text(text)), ..shape(id, name) };
+    a.shapes = vec![tb(1, "TextBox 1", 0.0, "Gone")];
+    b.shapes = vec![tb(2, "TextBox 2", 400.0, "New")];
+    assert!(morph_pairs(&a, &b).is_empty());
+    let f = morph_frame(&p, &a, &b, 0.5, &|_| false);
+    let xs: Vec<(&str, f64)> = f.shapes.iter().map(|(s, _)| (s.name.as_str(), s.xfrm.map_or(f64::NAN, |x| x.x))).collect();
+    assert_eq!(xs, [("TextBox 1", 0.0), ("TextBox 2", 400.0)]);
+    // Empty shapes of one kind and geometry still pair, and a named text box keeps its partner.
+    a.shapes.push(shape(3, "Rect A"));
+    b.shapes.push(shape(4, "Rect B"));
+    b.shapes[0].name = "TextBox 1".into();
+    assert_eq!(morph_pairs(&a, &b), [(ShapeId(1), ShapeId(2)), (ShapeId(3), ShapeId(4))]);
+}
+
 #[test]
 fn morph_xfrm_interpolates() {
     let a = Xfrm { rot: 350.0, ..Xfrm::new(0.0, 0.0, 100.0, 100.0) };
diff --git a/docs/animation-parity.md b/docs/animation-parity.md
index 1d50766b..74d246f1 100644
--- a/docs/animation-parity.md
+++ b/docs/animation-parity.md
@@ -24,7 +24,7 @@ three groups, with its Effect Options. All 48 round-trip through PPTX
 | Duration, advance on click / after time, Apply To All | yes | yes |
 | Transition sounds | yes | **no** (the model has the field; no picker, no playback) |
 | Preview in the editor | yes | yes |
-| Morph: objects move, resize, recolour | yes | yes |
+| Morph: objects move, resize, recolour; unmatched text boxes fade in place | yes | yes (#119) |
 | Morph: Words and Characters | yes | yes (#5) |
 | Morph: geometry (circle to star) | yes | yes (#27) |
 | Morph: `!!` name forcing, pictures, 3-D models, ink | yes | names partly; no 3-D or ink |
@@ -76,4 +76,5 @@ In, Wipe, Zoom, Float In, Grow/Shrink, Pulse, Spin, motion lines): weighted, ~85
 
 | Date | Change | Summary |
 |---|---|---|
+| 2026-10-11 | minor | Morph leaves unrelated text boxes unpaired: they fade in place (#119) |
 | 2026-10-10 | major | Created: 48/48 transitions, 64 of ~190 animation effects, pane and option depth, estimates |