ComboBox throws ReadOnlyException in case of change value during some long request

Originally by artamonov

I have button that performs some long operation and ComboBox. Button sets readonly true to ComboBox and performs long operation synchronously. During operation user can change value of ComboBox, in this case ReadOnlyException will be thrown.

If I use TextField instead of ComboBox then this case works fine. I have compared source code of AbtractTextField#changeVariables and ComboBox#changeVariables, there is additional check for !isReadOnly in AbstractTextField:185

if (variables.containsKey("text") && !isReadOnly()) {

I have tried this check with our custom ComboBox and this works fine. TestUI attached.

Steps to reproduce:

Click on Test button

Long request will start

Click on ComboBox drop down button (may be twice)

Select new value

Red indicator with ugly stacktrace on ComboBox will be shown

Imported from https://dev.vaadin.com/ issue #19238

Compare ComboBox#changeVariables with AbstractTextField#changeVariables, focusing on the read-only handling described in the issue. Reproduce the case with the attached TestUI by starting the long request and changing the ComboBox; done means the interaction no longer produces a ReadOnlyException or red error indicator.