Change external annotations location in IDEA 2019.3+

Question

IntelliJ IDEA lets you add external annotations.

However, the files go straight into the project's root directory.

How do I change the folder?
I can't find a setting.

Answer 1

This behavior is extremely confusing, and took me a long IDEA debug session to understand.
Let's say we start with a clean situation, and we add an external Annotation to a method of a library.

A popup appears, letting us choose the external Annotations root directory.

And we choose untitled3, the project's root directory. But ops, we made a mistake!
Now how can we change this directory? Where is it stored?
Well, actually not under any $projectRoot$/.idea/* XML file...

Let's debug IDEA. The "call" hierarchy is:

> AnnotateIntentionAction
  > AddAnnotationPsiFix#invoke
    > ExternalAnnotationsManagerImpl#annotateExternally

The last method annotateExternally calls AnnotationOrderRootType.getFiles(entry), where entry is a LibraryOrderEntry.

Here orderEntry.getRootFiles(getInstance()) gets called and the resulting VirtualFile(s) are used as library's external Annotations roots.

Where are they stored?

> Project structure
  > Modules
    > *your main module with dependencies*
      > *your library*
        > DOUBLE LEFT CLICK

Now press CANC over the Annotations entry, and you're done... Finally.
You can restart over.

Answer 2

You may drag'n'drop the annotations.xml file in the Project pane and move it to the desired location or change the path explicitly in Project Structure | Project Settings | Modules -> module Paths | External Annotations