How to obtain the embedded editor that shows when using VcsDiffUtil.showDiffFor()

You can register `com.intellij.diff.DiffExtension`, cast passed `DiffViewer` to `TwosideTextDiffViewer` and use `getEditor1()/getEditor2()` methods to get these editors.

If you need to differentiate this particular diff window from others, you'll have to create it yourself.
For example, using `com.intellij.openapi.vcs.changes.actions.diff.ShowDiffAction#showDiffForChange` and passing extra data via `ShowDiffContext.putChainContext` (that can be later retrieved in `DiffExtension` via `DiffContext.getUserData`).

That was very helpful! I will look to incorporate your suggestions. Thanks! 

So after looking into registering DiffExtensions how do I actually do it? I have seen other projects using diffManager.registerDiffTool() but that class seem to have been deprecated. 

>how do I actually do it
How do you register it? Same way as with other extensions.
https://www.jetbrains.org/intellij/sdk/docs/basics/plugin_structure/plugin_extensions_and_extension_points.html

Thank you Aleksey! For anyone else searching the forum and trying to figure out how to get a hook into the editors in the diff window this is how you do it (implementation in Scala)

object MyDiffExtension {
  val MY_CONTEXT_KEY: Key[Boolean] = KeyWithDefaultValue.create("MY_CONTEXT_KEY", false)
}

class MYDiffExtension extends DiffExtension {


  override def onViewerCreated(frameDiff: FrameDiffTool.DiffViewer,
                               diffContext: DiffContext,
                               diffRequest: DiffRequest): Unit = {
    val isMyDiffTool: Boolean =
      diffContext.getUserData(CruxDiffExtension.MY_CONTEXT_KEY)

    // Return if the context in which the diffExtension was
    // called with my custom context
    if(!isMyDiffTool)
        return

    println("MYExtensionInitiated")

    val editor: Editor = frameDiff.asInstanceOf[TwosideTextDiffViewer]
      .getEditor2
}}}

Elsewhere in your plugin.xml file you merely register this extension as follows:

<extensions defaultExtensionNs="com.intellij">
    <diff.DiffExtension implementation="MyDiffExtension"/>
</extensions>

Now you can do 

diffContext.putChainContext(MyDiffExtension.MY_CONTEXT_KEY, true)

ShowDiffAction.showDiffForChange(project, changes, 0, diffContext)

This will trigger the Diff View with your appropriate extension hooked in when it matches the context.

Hope this helps others!