Experiment with an empty delimiter heuristc on changed lists

See #587

a1.py

@@ -0,0 +1,5 @@
+def test(a, b):
+    if a:
+        print(a)
+        if b:
+            print(b)

a2.py

@@ -0,0 +1,5 @@
+def test(a, b):
+    if a:
+        print(a)
+    if b:
+        print(b)

sample_files/compare.expected

@@ -53,7 +53,7 @@ sample_files/css_1.css sample_files/css_2.css
 4b7101d68f953344af5c8839e7b13a56  -
 
 sample_files/dart_1.dart sample_files/dart_2.dart
-631b39f8574990923d4ac3d541e2966d  -
+84c93c4ee4794c1164e0ccbe9796ea2a  -
 
 sample_files/devicetree_1.dts sample_files/devicetree_2.dts
 77f13f1cff53b996778d142a94fe58f6  -
@@ -65,7 +65,7 @@ sample_files/elisp_contiguous_1.el sample_files/elisp_contiguous_2.el
 beaf7d6c5136d3db7a36ff49a131b251  -
 
 sample_files/elm_1.elm sample_files/elm_2.elm
-33b71893107538cff574276f2837adbb  -
+75b2883821ac30f35707ecbf868927fa  -
 
 sample_files/elvish_1.elv sample_files/elvish_2.elv
 f80b47646e7dd2bd3a49393d00657465  -
@@ -83,13 +83,13 @@ sample_files/hare_1.ha sample_files/hare_2.ha
 6ac97614325dd9b7ebb2b3cc1e4d3296  -
 
 sample_files/haskell_1.hs sample_files/haskell_2.hs
-9e6e08f67a41c963250bc1748c4824ce  -
+b74430633efe6be3bf6330f6d8ab824f  -
 
 sample_files/hcl_1.hcl sample_files/hcl_2.hcl
 4c3dc042f8cd57fec0ea883274b5f94e  -
 
 sample_files/hello_world_1.smali sample_files/hello_world_2.smali
-296286b067e86cccfad1fa9a0440993b  -
+570555eaaada48189d921573a06b7a72  -
 
 sample_files/helpful_1.el sample_files/helpful_2.el
 056471124ae2e582942e214ce9222477  -
@@ -128,13 +128,13 @@ sample_files/json_1.json sample_files/json_2.json
 d2c1aea9ec9be1078e4b70be4c9e1b13  -
 
 sample_files/jsx_1.jsx sample_files/jsx_2.jsx
-712c1c17f17f64745395e3d558ab53f3  -
+2ab44912902865a6d408c6998bcd39ac  -
 
 sample_files/julia_1.jl sample_files/julia_2.jl
 9782f864fc462c9f873760f71c43c48d  -
 
 sample_files/load_1.js sample_files/load_2.js
-22365043359164f717d495336d86b996  -
+557d6a9f82a116e75c22be5419053a5d  -
 
 sample_files/long_line_1.txt sample_files/long_line_2.txt
 7fc50bd547f0c20fda89a1931e5eb61e  -
@@ -161,13 +161,13 @@ sample_files/multiline_string_1.ml sample_files/multiline_string_2.ml
 d8d0b40bad1697ca143709d5ab46869f  -
 
 sample_files/nest_1.rs sample_files/nest_2.rs
-d3a799fe2cd9d81aa251c96af5cd9711  -
+bc148be13151c0eda4b4ebde0c44f2d7  -
 
 sample_files/nested_slider_1.el sample_files/nested_slider_2.el
 232c0736336031d762a1453f323e8064  -
 
 sample_files/nested_slider_1.rs sample_files/nested_slider_2.rs
-e9045007a25352eb81655458aebb452c  -
+77b17c0b44ddcc1fd108511984fd720f  -
 
 sample_files/nesting_1.el sample_files/nesting_2.el
 fe12489ab4bb156c0aee478dd36f264a  -
@@ -206,10 +206,10 @@ sample_files/preprocesor_1.h sample_files/preprocesor_2.h
 a680158a7980d738ba831aab3a63165d  -
 
 sample_files/qml_1.qml sample_files/qml_2.qml
-c23bda1d924adb57eb9e5c20744e99ea  -
+98624e3aaad26b552c6d74db37ddb039  -
 
 sample_files/r_1.R sample_files/r_2.R
-10f45a80a8554419bf30a2a0f574ab86  -
+cd55e900a72253e1bba6212d0dd3261d  -
 
 sample_files/racket_1.rkt sample_files/racket_2.rkt
 605aec93fa7b89d08e5d8ed56ad3c1be  -
@@ -239,7 +239,7 @@ sample_files/slider_at_end_1.json sample_files/slider_at_end_2.json
 cb370f1c0ccc5e155743330629f899f0  -
 
 sample_files/slow_1.rs sample_files/slow_2.rs
-2e76c128e008d5d255fdd29562208abc  -
+09fde8fdc933ebb95c7dec36fe9a53ff  -
 
 sample_files/small_1.js sample_files/small_2.js
 6bd2d5c9e2b843cfe0748355f3203de9  -
@@ -269,7 +269,7 @@ sample_files/text_1.txt sample_files/text_2.txt
 87914b8ef4a6b042054633ddb182ce02  -
 
 sample_files/todomvc_1.gleam sample_files/todomvc_2.gleam
-0a6f8335944adbd779c93aa52b7a30c5  -
+723951e8d4b377a595a8392f91e1375f  -
 
 sample_files/toml_1.toml sample_files/toml_2.toml
 df36cf07bb1b56fbdbcea7f651b4344d  -
@@ -281,13 +281,13 @@ sample_files/typescript_1.ts sample_files/typescript_2.ts
 fee7ee33d2037ad1941ba6bb5532a1db  -
 
 sample_files/typing_1.ml sample_files/typing_2.ml
-36161bd77a8c86643bc90656ec41c92c  -
+80f23aba9aa1bfe0460855451ecde74c  -
 
 sample_files/utf16_1.py sample_files/utf16_2.py
 39014a682ed2318f980c7ea4177cf659  -
 
 sample_files/vhdl_1.vhd sample_files/vhdl_2.vhd
-0f4de895a568092795c87f3d0a8a07e8  -
+9ef53ef84f9a750b01db0ef6bde254cc  -
 
 sample_files/whitespace_1.tsx sample_files/whitespace_2.tsx
 ac8b1a89ac26333f2d4e9433b2ca3958  -

src/diff/sliders.rs

@@ -32,9 +32,15 @@
 use line_numbers::SingleLineSpan;
 
 use crate::{
-    diff::changes::{insert_deep_novel, insert_deep_unchanged, ChangeKind::*, ChangeMap},
-    parse::guess_language,
-    parse::syntax::Syntax::{self, *},
+    diff::changes::{
+        insert_deep_novel, insert_deep_unchanged,
+        ChangeKind::{self, *},
+        ChangeMap,
+    },
+    parse::{
+        guess_language,
+        syntax::Syntax::{self, *},
+    },
 };
 
 pub(crate) fn fix_all_sliders<'a>(
@@ -47,6 +53,58 @@ pub(crate) fn fix_all_sliders<'a>(
     fix_all_sliders_one_step(nodes, change_map);
 
     fix_all_nested_sliders(language, nodes, change_map);
+
+    fix_invisible_delims(nodes, change_map);
+}
+
+fn fix_invisible_delims<'a>(nodes: &[&'a Syntax<'a>], change_map: &mut ChangeMap<'a>) {
+    for node in nodes {
+        match node {
+            List {
+                open_content,
+                children,
+                close_content,
+                ..
+            } => {
+                if let Some(change_kind) = change_map.get(node) {
+                    if open_content == ""
+                        && close_content == ""
+                        && matches!(change_kind, ChangeKind::Novel)
+                    {
+                        let mut all_children_unchanged = true;
+                        for child in children {
+                            let Some(child_change) = change_map.get(child) else {
+                                continue;
+                            };
+
+                            if !matches!(child_change, ChangeKind::Unchanged(_)) {
+                                all_children_unchanged = false;
+                                break;
+                            }
+                        }
+
+                        if all_children_unchanged {
+                            for child in children {
+                                let Some(child_change) = change_map.get(child) else {
+                                    continue;
+                                };
+
+                                let Unchanged(opposite_child) = child_change else {
+                                    continue;
+                                };
+
+                                change_map.insert(child, ChangeKind::Novel);
+                                change_map.insert(opposite_child, ChangeKind::Novel);
+                            }
+                        }
+                    } else {
+                        fix_invisible_delims(children, change_map);
+                    }
+                }
+            }
+            Atom { .. } => {}
+        }
+    }
 }
 
 /// Should nester slider correction prefer the inner or outer