Why is the JavaScript in my BUI customization no longer working?

Environment:

Agent Browser UI (BUI) Extensibility Framework, BUI updates

Issue:

Browser control custom script is broken after BUI Update

Resolution:

Browser UI (BUI) functionality should be customized using the BUI Extension Framework. If, rather, such customizations are developed using custom scripts embedded in workspace browser controls (that do not use the BUI Extension Framework), they are likely to break during a BUI update. The BUI workspace is updated weekly, and for further details see 

Answer ID 12588: BUI extension is failing to work after update

Answer ID 8598: Browser User Interface Updates

Answer ID 12699: Best Practices for BUI Extension Building