Optimization: Use absolute positioning instead of flexbox in editor

Flexbox was causing layouts and repaints to cover the entire scroll view instead of just the edited line. This cuts down on DOM manipulation cost significantly.

static/editor.css

@@ -12,16 +12,8 @@
   height: auto;
 }
 
-.editor .flexbox {
-  display: -webkit-box;
-  position: absolute;
-  top: 0;
-  bottom: 0;
-  left: 0;
-  right: 0;
-}
-
 .editor .gutter {
+  position: absolute;
   height: 100%;
   overflow: hidden;
   padding-left: 0.4em;
@@ -52,6 +44,11 @@
 }
 
 .editor .scroll-view {
+  position: absolute;
+  top: 0;
+  bottom: 0;
+  left: 0;
+  right: 0;
   overflow-x: auto;
   overflow-y: hidden;
   -webkit-box-flex: 1;