Form is in a broken state (example - right click menu options, related lists, reference fields etc are unavailable) as a result of "Uncaught TypeError: Cannot read property 'style' of undefined" browser console error
UI Policy script contains code referencing objects that do not exist
Customer may have a customized UI Policy which contains code similar to below in the Scripts section of the UI Policy:
var sections = g_form.getSections();
sections.style.display = 'block';
In case the form does not have a Section the "Uncaught TypeError: Cannot read property 'style' of undefined" browser console error will get thrown and cause the issue.
Removing the offensive code or de-activating the UI Policy will resolve the issue.