Merge branch '2.3' into 2.4

CHANGELOG.md

 # Changelog
 
+## 2.3.4
+Bug fixes:
+* Fix layout problems in Firefox 49+
+* ARSnova no longer hangs because of a `ReferenceError` when loading for the
+  first time on slow connections.
+
 ## 2.4.3
 Bug fixes:
 * Buttons of the question format bar are now correctly shown or hidden when

src/main/webapp/resources/sass/app/_bugfixes.scss

@@ -65,3 +65,20 @@ body.x-android.splashscreenClosed {
 body.x-windows {
 	touch-action: none !important;
 }
+
+body.x-firefox {
+	/* Override bad CSS code of Sencha Touch which breaks layout in Firefox 49+.
+	 * See https://hacks.mozilla.org/2016/09/firefox-49-fixes-sites-designed-with-webkit-in-mind-and-more/
+	 */
+	.x-button, .x-layout-box {
+		display: flex;
+	}
+
+	.x-layout-box > .x-layout-box-item {
+		display: flex !important;
+	}
+
+	.x-stretched.x-container > .x-inner, .x-stretched.x-container > .x-body, .x-stretched.x-container > .x-body > .x-inner {
+		display: flex !important;
+	}
+}