Mixins were not the only bridges. Bridges must actually not be final.

Author: sjrd

compiler/src/dotty/tools/backend/jvm/BTypesFromSymbols.scala

@@ -298,7 +298,9 @@ class BTypesFromSymbols[I <: DottyBackendInterface](val int: I, val frontendAcce
         // without having to provide any implementations, but that is an
         // illegal combination of modifiers at the bytecode level so
         // suppress final if abstract if present.
-        && !sym.isOneOf(AbstractOrTrait), ACC_FINAL)
+        && !sym.isOneOf(AbstractOrTrait)
+        // Bridges can be final, but final bridges confuse some frameworks
+        && !sym.is(Bridge), ACC_FINAL)
       .addFlagIf(sym.isStaticMember, ACC_STATIC)
       .addFlagIf(sym.is(Bridge), ACC_BRIDGE | ACC_SYNTHETIC)
       .addFlagIf(sym.is(Artifact), ACC_SYNTHETIC)

tests/run/mixin-final-def-object-lucre.scala

@@ -0,0 +1,19 @@
+trait EventLike
+
+trait GrandParent:
+  def changed: EventLike
+
+trait HasChanged extends GrandParent:
+  override def changed: EventLike
+
+abstract class Parent extends GrandParent:
+  object changed extends EventLike
+
+class Child extends Parent with HasChanged
+
+object Test:
+  def main(args: Array[String]): Unit =
+    val child = Child()
+    println(child.changed)
+    println((child: HasChanged).changed)
+end Test