Revert "remove flex from splitview"

This reverts commit 4db611a4.

src/vs/base/browser/ui/splitview/splitview.css

@@ -21,13 +21,23 @@
 }
 
 .monaco-split-view2 > .split-view-container {
+	display: flex;
 	width: 100%;
 	height: 100%;
 	white-space: nowrap;
 }
 
+.monaco-split-view2.vertical > .split-view-container {
+	flex-direction: column;
+}
+
+.monaco-split-view2.horizontal > .split-view-container {
+	flex-direction: row;
+}
+
 .monaco-split-view2 > .split-view-container > .split-view-view {
 	white-space: initial;
+	flex: none;
 	position: relative;
 }