fix(text-minimessage): Preserve non-text components in color changing tags

text-minimessage/src/main/java/net/kyori/adventure/text/minimessage/tag/standard/AbstractColorChangingTag.java

@@ -55,6 +55,11 @@
  */
 abstract class AbstractColorChangingTag implements Modifying, Examinable {
 
+  private static final ComponentFlattener LENGTH_CALCULATOR = ComponentFlattener.builder()
+    .mapper(TextComponent.class, TextComponent::content)
+    .unknownMapper(x -> "_") // every unknown component gets a single colour
+    .build();
+
   private boolean visited;
   private int size = 0;
   private int disableApplyingColorDepth = -1;
@@ -76,7 +81,7 @@ public final void visit(final @NotNull Node current, final int depth) {
       final TagNode tag = (TagNode) current;
       if (tag.tag() instanceof Inserting) {
         // ComponentTransformation.apply() returns the value of the component placeholder
-        ComponentFlattener.textOnly().flatten(((Inserting) tag.tag()).value(), s -> this.size += s.codePointCount(0, s.length()));
+        LENGTH_CALCULATOR.flatten(((Inserting) tag.tag()).value(), s -> this.size += s.codePointCount(0, s.length()));
       }
     }
   }
@@ -124,6 +129,10 @@ public final Component apply(final @NotNull Component current, final int depth)
       }
 
       return parent.build();
+    } else if (!(current instanceof TextComponent)) {
+      final Component ret = current.children(Collections.emptyList()).colorIfAbsent(this.color());
+      this.advanceColor();
+      return ret;
     }
 
     return Component.empty().mergeStyle(current);

text-minimessage/src/test/java/net/kyori/adventure/text/minimessage/tag/standard/GradientTagTest.java

@@ -32,6 +32,7 @@
 
 import static net.kyori.adventure.text.Component.empty;
 import static net.kyori.adventure.text.Component.text;
+import static net.kyori.adventure.text.Component.translatable;
 import static net.kyori.adventure.text.event.HoverEvent.showText;
 import static net.kyori.adventure.text.format.NamedTextColor.BLACK;
 import static net.kyori.adventure.text.format.NamedTextColor.BLUE;
@@ -542,4 +543,18 @@ void testDecorationsPreserved() {
 
     this.assertParsedEquals(expected, input, Placeholder.component("placeholder", placeholder));
   }
+
+  // https://github.com/KyoriPowered/adventure/issues/827
+  @Test
+  void testLangTagInGradient() {
+    final String input = "<gradient:red:blue>ab<lang:block.minecraft.diamond_block>!</gradient>";
+    final Component expected = Component.textOfChildren(
+      text("a", RED),
+      text("b", color(0xd55580)),
+      translatable("block.minecraft.diamond_block", color(0xaa55aa))
+        .append(text("!", color(0x8055d5)))
+    );
+
+    this.assertParsedEquals(expected, input);
+  }
 }