src/vs/workbench/browser/parts/editor/sideBySideEditor.ts/SideBySideEditor/onDidFocusChange
class SideBySideEditor:	private onDidFocusChange(side: Side.PRIMARY | Side.SECONDARY): void {
		this.lastFocusedSide = side;

		// Signal to outside that our active control changed
		this._onDidChangeControl.fire();
	}