Don't emit STAGE_SIZE_CHANGED if stage size didn't change

and also fix the other STAGE_SIZE_CHANGED event test to
actually test that it is being fired when there is a change

src/engine/runtime.js

@@ -2719,8 +2719,9 @@ class Runtime extends EventEmitter {
                     height / 2
                 );
             }
+
+            this.emit(Runtime.STAGE_SIZE_CHANGED, width, height);
         }
-        this.emit(Runtime.STAGE_SIZE_CHANGED, width, height);
     }
 
     // eslint-disable-next-line no-unused-vars

test/unit/engine_runtime_tw.js

@@ -171,6 +171,7 @@ test('setStageSize preserves monitor position relative to center of stage', t =>
 });
 
 test('setStageSize argument range', t => {
+    t.plan(6);
     const rt = new Runtime();
 
     rt.once('STAGE_SIZE_CHANGED', (width, height) => {
@@ -194,6 +195,15 @@ test('setStageSize argument range', t => {
     t.end();
 });
 
+test('STAGE_SIZE_CHANGED does not fire if no change', t => {
+    const rt = new Runtime();
+    rt.on('STAGE_SIZE_CHANGED', () => {
+        t.fail('STAGE_SIZE_CHANGED emitted');
+    });
+    rt.setStageSize(rt.stageWidth, rt.stageHeight);
+    t.end();
+});
+
 test('getNumberOfCloudVariables', t => {
     const rt = new Runtime();