gistfile1.txt

diff --git a/dom/bindings/Codegen.py b/dom/bindings/Codegen.py
index d33973d9..a1a229e 100644
--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -3240,7 +3240,7 @@ class CGConstructorEnabled(CGAbstractMethod):
         conditions = []
         iface = self.descriptor.interface
 
-        if not iface.isExposedInWindow():
+        if not iface.isExposedOnMainThread():
             exposedInWindowCheck = dedent(
                 """
                 MOZ_ASSERT(!NS_IsMainThread(), "Why did we even get called?");
@@ -3259,7 +3259,7 @@ class CGConstructorEnabled(CGAbstractMethod):
                 }
                 """, workerCondition=workerCondition.define())
             exposedInWorkerCheck = CGGeneric(exposedInWorkerCheck)
-            if iface.isExposedInWindow():
+            if iface.isExposedOnMainThread():
                 exposedInWorkerCheck = CGIfWrapper(exposedInWorkerCheck,
                                                    "!NS_IsMainThread()")
             body.append(exposedInWorkerCheck)
diff --git a/dom/bindings/parser/WebIDL.py b/dom/bindings/parser/WebIDL.py
index e664988..a2a83ce 100644
--- a/dom/bindings/parser/WebIDL.py
+++ b/dom/bindings/parser/WebIDL.py
@@ -501,6 +501,10 @@ class IDLExposureMixins():
     def isExposedInWindow(self):
         return 'Window' in self.exposureSet
 
+    def isExposedOnMainThread(self):
+        return 'Window' in self.exposureSet or
+               'System' in self.exposureSet
+
     def isExposedInAnyWorker(self):
         return len(self.getWorkerExposureSet()) > 0