Only set _preferredContentSizeCategory when the entire UI is updated; otherwise all calls to -updateFontsInViewsForPreferredContentSizeCategory: would set that flag which could cause problems when providing a preview of text size changes.
-endTrackingDynamicType was ineffective at preventing changes in UIWebViews and other components that may be observing the content size change notifications while previewing a new font size. Added -DTK_beginSilencingPreferredContentSizeChangeNotifications on UIApplication to allow setting the content size for the purpose of a preview without emitting notifications.