fix: correct time-header-gutter (jquense#2219)

make sure the header gutter is measured properly

Author: cutterbl

src/TimeGrid.js

@@ -29,14 +29,13 @@ export default class TimeGrid extends Component {
   getSnapshotBeforeUpdate() {
     this.calculateScroll()
     this.checkOverflow()
-
-    if (this.props.width == null) {
-      this.measureGutter()
-    }
     return null
   }
 
   componentDidMount() {
+    if (this.props.width == null) {
+      this.measureGutter()
+    }
     this.applyScroll()
 
     window.addEventListener('resize', this.handleResize)

src/sass/time-column.scss

@@ -110,7 +110,7 @@
   }
 
   .rbc-header,
-  .rbc-day-bg, {
+  .rbc-day-bg {
     width: 140px;
     // min-width: 0;
     flex:  1 1 0;