Prevent double scrollbars for flexbox parents

Previously attempting to use SimpleBar in a parent with "display: flex"
without specifying a height or "overflow: hidden" on the host element,
resulted in two scroll bars being displayed SimpleBar and the native one.

Fixes Grsmto#473.

packages/simplebar/src/simplebar.js

@@ -64,6 +64,14 @@ export default class SimpleBar {
 
     SimpleBar.getRtlHelpers = memoize(SimpleBar.getRtlHelpers);
 
+    const elWindow = getElementWindow(this.el);
+    const elOverflowY = elWindow.getComputedStyle(this.el).overflowY;
+    const parentDisplay = elWindow.getComputedStyle(this.el.parentElement).display;
+    if ((elOverflowY == 'auto' || elOverflowY == 'scroll')
+      && (parentDisplay == 'flex' || parentDisplay == 'inline-flex')) {
+      // Prevent bug where Simplebar is shown along with the native scrollbar.
+      this.el.style.height = '100%';
+    }
     this.init();
   }