Do not attempt to sub-diff multiline strings (#304)

Inspired by @gschlager 's PR; see
https://github.com/splitwise/super_diff/pull/301/changes#r2996407601.

Author: jas14

CHANGELOG.md

@@ -10,6 +10,7 @@
 - Support elision for flat line trees. [#300](https://github.com/splitwise/super_diff/pull/300) by [@gschlager](https://github.com/gschlager)
 - Fix integration snippet indentation. [#299](https://github.com/splitwise/super_diff/pull/299) by [@gschlager](https://github.com/gschlager)
 - Pin all actions to full commit SHA. [#305](https://github.com/splitwise/super_diff/pull/305)
+- Do not attempt to sub-diff multiline strings. [#304](https://github.com/splitwise/super_diff/pull/304)
 
 ## 0.18.0 - 2025-12-05
 

lib/super_diff/basic/operation_tree_builders/multiline_string.rb

@@ -43,6 +43,13 @@ def build_operation_tree
 
         attr_reader :sequence_matcher, :original_expected, :original_actual
 
+        # override
+        def should_compare?(_operation, _next_operation)
+          # Don't try to build a nested operation tree for individual line changes, even if
+          # there's an applicable string operation tree builder.
+          false
+        end
+
         def split_into_lines(string)
           string.scan(/.*(?:\r|\n|\r\n|\Z)/)
         end

spec/unit/basic/operation_tree_builders/multiline_string_spec.rb

@@ -2,6 +2,41 @@
 
 require 'spec_helper'
 
+class WordOperationTreeBuilder < SuperDiff::Core::AbstractOperationTreeBuilder
+  def self.applies_to?(expected, actual)
+    expected.is_a?(::String) && actual.is_a?(::String)
+  end
+
+  protected
+
+  def unary_operations
+    expected_words = expected.split
+    actual_words = actual.split
+
+    Diff::LCS.diff(expected_words, actual_words).flat_map do |change_group|
+      change_group.map do |change|
+        SuperDiff::Core::UnaryOperation.new(
+          name: change.action == '+' ? :insert : :delete,
+          collection: expected,
+          key: change.position,
+          index: change.position,
+          value: change.element
+        )
+      end
+    end
+  end
+
+  def build_operation_tree
+    SuperDiff::Basic::OperationTrees::Array.new([])
+  end
+
+  private
+
+  def should_compare?(_operation, _next_operation)
+    false
+  end
+end
+
 RSpec.describe SuperDiff, type: :unit do
   describe '.diff' do
     subject(:diff) { SuperDiff.diff(expected, actual) }
@@ -115,5 +150,33 @@
         expect(diff).to eq(expected_output)
       end
     end
+
+    context 'when a string operation tree builder exists' do
+      around do |example|
+        with_configuration(extra_operation_tree_builder_classes: SuperDiff.configuration.extra_operation_tree_builder_classes + [WordOperationTreeBuilder]) do
+          example.run
+        end
+      end
+
+      let(:actual) { <<~STRING }
+        This here is a string.
+        It contains separate lines.
+        This one is different.
+      STRING
+
+      it 'does not attempt to diff each line' do
+        expected_output =
+          SuperDiff::Core::Helpers
+          .style(color_enabled: true) do
+            plain_line '  This here is a string.\\n'
+            plain_line '  It contains separate lines.\\n'
+            expected_line '- What else can I say?\\n'
+            actual_line '+ This one is different.\\n'
+          end
+            .to_s
+            .chomp
+        expect(diff).to eq(expected_output)
+      end
+    end
   end
 end