Templating: Fix multi-select keyboard toggle bug (#26841)

Fixes #20953

public/app/features/variables/pickers/shared/VariableInput.tsx

@@ -12,6 +12,7 @@ export class VariableInput extends PureComponent<Props> {
     if (NavigationKey[event.keyCode]) {
       const clearOthers = event.ctrlKey || event.metaKey || event.shiftKey;
       this.props.onNavigate(event.keyCode as NavigationKey, clearOthers);
+      event.preventDefault();
     }
   };