Changing widget_display_callback hook priority.

What does widget_display_callback hook priority actually do and will it create any performance issues by having it at “1”?

This hook is used to launch the function which determines if the current widgets is a fixed widget. The hook priority just saying “do it early” or “do it later”. So no performance imacted here!

I got it working again bty changing “widget_display_callback hook priority:” from 50 to 1 (it did not work with 10 nor 20 and before it worked with 50).

I suspect that you have other plugins on this hook. So not the core update to 4.0 coused the problem, but the parallel update of those plugins!.. Just a theory )