src/vs/workbench/browser/parts/editor/sideBySideEditor.ts/SideBySideEditor/getSelection
class SideBySideEditor:	getSelection(): IEditorPaneSelection | undefined {
		const lastFocusedEditorPane = this.getLastFocusedEditorPane();
		if (isEditorPaneWithSelection(lastFocusedEditorPane)) {
			const selection = lastFocusedEditorPane.getSelection();
			if (selection) {
				return new SideBySideAwareEditorPaneSelection(selection, lastFocusedEditorPane === this.primaryEditorPane ? Side.PRIMARY : Side.SECONDARY);
			}
		}

		return undefined;
	}