Fix Breadcrumbs2 ending up with 2 tabIndexes on Firefox due to explicit css overflow scroll

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/structures/AutoHideScrollbar.js

@@ -43,6 +43,7 @@ export default class AutoHideScrollbar extends React.Component {
             className={["mx_AutoHideScrollbar", this.props.className].join(" ")}
             onScroll={this.props.onScroll}
             onWheel={this.props.onWheel}
+            tabIndex={this.props.tabIndex}
         >
             { this.props.children }
         </div>);

src/components/structures/LeftPanel2.tsx

@@ -327,6 +327,9 @@ export default class LeftPanel2 extends React.Component<IProps, IState> {
                 <IndicatorScrollbar
                     className="mx_LeftPanel2_breadcrumbsContainer mx_AutoHideScrollbar"
                     verticalScrollsHorizontally={true}
+                    // Firefox sometimes makes this element focusable due to
+                    // overflow:scroll;, so force it out of tab order.
+                    tabIndex={-1}
                 >
                     <RoomBreadcrumbs2 />
                 </IndicatorScrollbar>