a11y: do not eat the Tab key in iframe editors

src/vs/workbench/browser/parts/editor/iframeEditor.ts

@@ -137,7 +137,7 @@ export class IFrameEditor extends BaseEditor {
 	private enableKeybindings(): string {
 		return [
 			'<script>',
-			'var ignoredKeys = [32 /* space */, 33 /* page up */, 34 /* page down */, 38 /* up */, 40 /* down */];',
+			'var ignoredKeys = [9 /* tab */, 32 /* space */, 33 /* page up */, 34 /* page down */, 38 /* up */, 40 /* down */];',
 			'var ignoredCtrlCmdKeys = [67 /* c */];',
 			'window.document.body.addEventListener("keydown", function(event) {',		// Listen to keydown events in the iframe
 			'	try {',