The new form rendering engine - The new form renderer was built to provide better performance. Forms will load significantly faster and more efficiently. The new renderer is based on previous generations and has the same functionality and behavior
- Iframes are now kept throughout the user session
- Custom scripts are loaded in separate iframes
- No changes in supported scripts or form
- Any attempt to access DOM in the content iframe using JS, jQuery or other 3rd party libraries (document.getElementById() or jQuery selectors)
- Creating a new HTML content in the parent window for persistent content (and assumed that the parent window was the main CRM iframe.
- Window.load, parsing iframe/form URL
- Attempting to use unsupported (non-XRM) APIs, especially undocumented ones that may have been shipped with CRM for internal usage only
- Accessing window.parent() from a web resource that may assume for example there’s a variable set in the current window context.
More detailed info here:
However there are some things that we need to do to ensure full compatibility when upgrading. So use the Custom Code Validation Tool, to validate your code before you update, use always a sandbox before you update the Production server.