src/vs/workbench/common/editor.ts/EditorResourceAccessorImpl/getSideEditors
class EditorResourceAccessorImpl:	private getSideEditors(editor: EditorInput | IUntypedEditorInput): { primary: EditorInput | IUntypedEditorInput | undefined; secondary: EditorInput | IUntypedEditorInput | undefined } {
		if (isSideBySideEditorInput(editor) || isResourceSideBySideEditorInput(editor)) {
			return { primary: editor.primary, secondary: editor.secondary };
		}

		if (isDiffEditorInput(editor) || isResourceDiffEditorInput(editor)) {
			return { primary: editor.modified, secondary: editor.original };
		}

		return { primary: undefined, secondary: undefined };
	}