Update containerBoundingRect onMouseDown

Update the containerBoundingRect on every mouseDown event. This ensures the container coords are correct on page resize and hopefully fixes issue agentcooper#259 .

This does add some extra overhead, but less than re-rendering the whole element on a page resize.

src/components/MouseSelection.tsx

@@ -81,9 +81,7 @@ class MouseSelection extends Component<Props, State> {
     let containerBoundingRect: DOMRect | null = null;
 
     const containerCoords = (pageX: number, pageY: number) => {
-      if (!containerBoundingRect) {
-        containerBoundingRect = container.getBoundingClientRect();
-      }
+      containerBoundingRect ||= container.getBoundingClientRect();
 
       return {
         x: pageX - containerBoundingRect.left + container.scrollLeft,
@@ -120,6 +118,7 @@ class MouseSelection extends Component<Props, State> {
       }
 
       onDragStart();
+      containerBoundingRect = container.getBoundingClientRect();
 
       this.setState({
         start: containerCoords(event.pageX, event.pageY),