Fixes a tiny dead zone because of the resizer

AFAICT, the `-webkit-app-region: no-drag` of the resizer seems to only be honored for the first 4px. Any more and it doesn't eat the mouse event and creates a dead zone

src/vs/workbench/browser/parts/titlebar/media/titlebarpart.css

@@ -68,7 +68,7 @@
 	position: absolute;
 	top: 0;
 	width: 100%;
-	height: 20%;
+	height: 4px;
 }
 
 .monaco-workbench.windows.fullscreen .part.titlebar > .resizer,