Expected Behavior

Custom classes can be added to widgets.

Actual Behavior

Custom classes (text or predefined) do not get written out to html, but are displayed in Widget UI. Even custom widget css classes built into theme no longer work. The standard ones do get written out to html successfully (widget-first, widget-last, etc)

Steps to Reproduce the Problem

1. Install dFactory Responsive Lightbox plugin v 1.7.2 ( https://wordpress.org/plugins/responsive-lightbox/ )
2. Goto Plugin Settings, and General Settings tab
3. Select either "PrettyPhoto" or "Featherlight" as Default Lightbox (only tested with these two and the paid "Lightcase" that I purchased)
4. Check the box (enable) for "Widgets : Enable lightbox for widgets content."
5. Click Save at bottom.
6. Clear browser cache, and reload site to see issue.
7. Tried deactivating and deleting Widget CSS Classes and reinstalling, but issue persisted.
8. Disabled Repsonsive Lightbox setting for widget contents to resolve.

Notes

• I use a page caching plugin, but cleared cache for all test steps.

Specifications

• Plugin Version: 1.5.2.1
• WordPress Version: 4.9.1
• Other plugins installed: many listed in attached image. Issue occurred on installing and configuring Responsive Lightbox
• Theme: Shaped Pixels v 1.1.2 ( http://shapedpixels.com )
• Browser: On Mac, tested with current versions of Firefox, Chrome and Safari