Drupal 8.x

In a form I have various fields and in several of these (fields) I have inserted callBacks to perform calculations.
Eg all subsequent fields have their own callBack:
Field 1 callBack 1
Field 2 callBack 2
Field 3 callBack 3
In sequence everything is fine. The problem arises when after typing the data in field 3 I return to field 1 to update a value. Since the value of field 1 is used in the calculation of the callBack of field 3, I am obliged to update the value of field 3 to have the calculation done again.

Hi guys,

I do have a contact webform sending a submission confirmation mail to the submitter to acknowledge we got their message and confirm we'll get in touch with them asap.

To prevent undesired replies at that stage, I would like to have that email sent from noreply@mydomain.net , that one being a blackhole address. However sending that email fails if address does not exist. It even throws an error in Drupal log but without real details beside "Error sending email ...."

I'm updating a D7 form for D8 and it makes a query to a public API for entities that could become Drupal nodes. Its performance is spotty, likely because as I see the Form API lifecycle that the form gets rebuilt as part of its security against hacks. And it seems to me that the re-run of the queries fail because of this.

Since the form is cached, could I cache the results of the first time the query is run so as to prevent timeout by the public API? Here's a subset of the form that is applicable.

Hello everyone,

I installed bootstrap4 theme (URL).
I created a subtheme : 
I went in Appearance tab, bootstrap4 setting, on the bottom of the page : 

Screenshot

Then I set my new subtheme as default.

When Bootstrap4 theme is set as default, my front page looks like this : Screenshot