Ensure incompatible rules get the correct vendor prefix

#509

src/lib.rs

@@ -9631,6 +9631,34 @@ mod tests {
         ..Browsers::default()
       },
     );
+
+    prefix_test(
+      r#"
+      *,
+      ::before,
+      ::after,
+      ::backdrop {
+        padding: 5px;
+      }
+      "#,
+      indoc! {r#"
+        *, :before, :after {
+          padding: 5px;
+        }
+
+        ::-webkit-backdrop {
+          padding: 5px;
+        }
+
+        ::backdrop {
+          padding: 5px;
+        }
+      "#},
+      Browsers {
+        safari: Some(14 << 16),
+        ..Browsers::default()
+      },
+    );
   }
 
   #[test]

src/rules/mod.rs

@@ -624,6 +624,8 @@ impl<'i, T: Clone> CssRuleList<'i, T> {
             smallvec![]
           };
 
+          style.update_prefix(context);
+
           // Attempt to merge the new rule with the last rule we added.
           let mut merged = false;
           if let Some(CssRule::Style(last_style_rule)) = rules.last_mut() {
@@ -659,6 +661,7 @@ impl<'i, T: Clone> CssRuleList<'i, T> {
               let list = SelectorList::new(smallvec![selector]);
               let mut clone = style.clone();
               clone.selectors = list;
+              clone.update_prefix(context);
 
               // Also add rules for logical properties and @supports overrides.
               let supports = context.handler_context.get_supports_rules(&clone);

src/rules/style.rs

@@ -84,11 +84,6 @@ impl<'i, T: Clone> StyleRule<'i, T> {
       }
     }
 
-    self.vendor_prefix = get_prefix(&self.selectors);
-    if self.vendor_prefix.contains(VendorPrefix::None) && context.targets.should_compile_selectors() {
-      self.vendor_prefix = downlevel_selectors(self.selectors.0.as_mut_slice(), *context.targets);
-    }
-
     Ok(false)
   }
 }
@@ -158,6 +153,13 @@ impl<'i, T> StyleRule<'i, T> {
         .zip(other_rule.declarations.iter())
         .all(|((a, _), (b, _))| a.property_id() == b.property_id())
   }
+
+  pub(crate) fn update_prefix(&mut self, context: &mut MinifyContext<'_, 'i>) {
+    self.vendor_prefix = get_prefix(&self.selectors);
+    if self.vendor_prefix.contains(VendorPrefix::None) && context.targets.should_compile_selectors() {
+      self.vendor_prefix = downlevel_selectors(self.selectors.0.as_mut_slice(), *context.targets);
+    }
+  }
 }
 
 fn parse_at<'i, 't, T, F>(