[Filter Bar] Remove flickering when opening filter bar popover (#56222) (#56385)

Co-authored-by: Elastic Machine <[email protected]>

Co-authored-by: Elastic Machine <[email protected]>

Author: Maja Grubic

src/plugins/data/public/ui/filter_bar/filter_bar.tsx

@@ -96,6 +96,7 @@ function FilterBarUI(props: Props) {
           withTitle
           panelPaddingSize="none"
           ownFocus={true}
+          initialFocus=".filterEditor__hiddenItem"
         >
           <EuiFlexItem grow={false}>
             <div style={{ width: 400 }}>

src/plugins/data/public/ui/filter_bar/filter_editor/index.tsx

@@ -95,6 +95,7 @@ class FilterEditorUI extends Component<Props, State> {
                 defaultMessage="Edit filter"
               />
             </EuiFlexItem>
+            <EuiFlexItem grow={false} className="filterEditor__hiddenItem" />
             <EuiFlexItem grow={false}>
               <EuiButtonEmpty size="xs" onClick={this.toggleCustomEditor}>
                 {this.state.isCustomEditorOpen ? (