Fix an edge case in superselector computation (#1866)

Closes #1843

Author: nex3

CHANGELOG.md

@@ -3,6 +3,11 @@
 * Remove sourcemap comments from Sass sources. The generated sourcemap comment
  for the compiled CSS output remains unaffected.
 
+* Fix a bug in `@extend` logic where certain selectors with three or more
+ combinators were incorrectly considered superselectors of similar selectors
+ with fewer combinators, causing them to be incorrectly trimmed from the
+ output.
+
 * Produce a better error message for a number with a leading `+` or `-`, a
  decimal point, but no digits.
 

lib/src/extend/functions.dart

@@ -598,6 +598,7 @@ bool complexIsSuperselector(List<ComplexSelectorComponent> complex1,
 
  var i1 = 0;
  var i2 = 0;
+ Combinator? previousCombinator;
  while (true) {
  var remaining1 = complex1.length - i1;
  var remaining2 = complex2.length - i2;
@@ -643,6 +644,11 @@ bool complexIsSuperselector(List<ComplexSelectorComponent> complex1,
  parents.add(component2);
  }
 
+ if (!_compatibleWithPreviousCombinator(
+ previousCombinator, parents ?? const [])) {
+ return false;
+ }
+
  var component2 = complex2[endOfSubselector];
  var combinator1 = component1.combinators.firstOrNull;
  var combinator2 = component2.combinators.firstOrNull;
@@ -652,6 +658,7 @@ bool complexIsSuperselector(List<ComplexSelectorComponent> complex1,
 
  i1++;
  i2 = endOfSubselector + 1;
+ previousCombinator = combinator1;
 
  if (complex1.length - i1 == 1) {
  if (combinator1 == Combinator.followingSibling) {
@@ -671,6 +678,25 @@ bool complexIsSuperselector(List<ComplexSelectorComponent> complex1,
  }
 }
 
+/// Returns whether [parents] are valid intersitial components between one
+/// complex superselector and another, given that the earlier complex
+/// superselector had the combinator [previous].
+bool _compatibleWithPreviousCombinator(
+ Combinator? previous, List<ComplexSelectorComponent> parents) {
+ if (parents.isEmpty) return true;
+ if (previous == null) return true;
+
+ // The child and next sibling combinators require that the *immediate*
+ // following component be a superslector.
+ if (previous != Combinator.followingSibling) return false;
+
+ // The following sibling combinator does allow intermediate components, but
+ // only if they're all siblings.
+ return parents.every((component) =>
+ component.combinators.firstOrNull == Combinator.followingSibling ||
+ component.combinators.firstOrNull == Combinator.nextSibling);
+}
+
 /// Returns whether [combinator1] is a supercombinator of [combinator2].
 ///
 /// That is, whether `X combinator1 Y` is a superselector of `X combinator2 Y`.