src/vs/workbench/common/editor/sideBySideEditorInput.ts/SideBySideEditorInput/resource
class SideBySideEditorInput:	get resource(): URI | undefined {
		if (this.hasIdenticalSides) {
			// pretend to be just primary side when being asked for a resource
			// in case both sides are the same. this can help when components
			// want to identify this input among others (e.g. in history).
			return this.primary.resource;
		}

		return undefined;
	}