src/vs/workbench/contrib/customEditor/browser/customEditors.ts/CustomEditorService/createDiffEditorInput
class CustomEditorService:	private createDiffEditorInput(
		editor: IResourceDiffEditorInput,
		editorID: string,
		group: IEditorGroup
	): DiffEditorInput {
		const createEditorForSubInput = (subInput: IResourceEditorInput | IUntitledTextResourceEditorInput, editorID: string, customClasses: string): EditorInput | undefined => {
			if (!subInput.resource) {
				return;
			}
			// We check before calling this call back that both resources are defined
			const input = CustomEditorInput.create(this.instantiationService, subInput.resource, editorID, group.id, { customClasses });
			return input instanceof EditorInput ? input : undefined;
		};

		const modifiedOverride = createEditorForSubInput(editor.modifiedInput, editorID, 'modified') ?? this.editorService.createEditorInput(editor.modifiedInput);
		const originalOverride = createEditorForSubInput(editor.originalInput, editorID, 'original') ?? this.editorService.createEditorInput(editor.originalInput);

		return this.instantiationService.createInstance(DiffEditorInput, undefined, undefined, originalOverride, modifiedOverride, true);
	}