Since silverstripe/silverstripe-admin version 1.11 the include `SilverStripe\\Admin\\LeftAndMain_ViewModeSelector` uses an [ExtraClass parameter](6d1998707d/templates/SilverStripe/Admin/Includes/LeftAndMain_ViewModeSelector.ss (L1)). When this parameter is not explicitly set it inherits the ExtraClass of the template it's included in. In CMSMain this causes the ViewMode button to stretch.