src/vs/workbench/browser/parts/editor/sideBySideEditor.ts/SideBySideEditor/getLastFocusedEditorPane
class SideBySideEditor:	private getLastFocusedEditorPane(): EditorPane | undefined {
		if (this.lastFocusedSide === Side.SECONDARY) {
			return this.secondaryEditorPane;
		}

		return this.primaryEditorPane;
	}