Add CSS class to widget when fetching data from backend

[Alex-Sichkar]

When a widget fetches data from the backend (e.g., when switching between time periods in a calendar), add a CSS class to indicate that the operation is in progress, so it can be styled accordingly. This is especially useful in cases of slow server responses.

[jrief]

What kind of proposal do you have for this?

[Alex-Sichkar]

Simply add something like the df-loading CSS class to the widget when it's fetching data from the backend, and remove it once done.

[Alex-Sichkar]

Maybe it also makes sense to implement a formset state indicator by applying CSS classes to the django-formset element. For example, use df-initialized when the formset is fully initialized and df-busy when a request is in progress. This could help reduce widget flickering (as was fixed for RTA and the date picker), although other widgets may still experience flickering. In that case, applying the CSS visibility property to the whole element might be a useful trick. Alternatively, it might be even better to emit global events for the lifecycle from the JS code for more flexible customization.