Deciphering Refactoring Branch Dynamics in Modern Code Review: An Empirical Study on Qt

Context: Modern code review is a widely employed technique in both industrial and open-source projects, serving to enhance software quality, share knowledge, and ensure compliance with coding standards and guidelines. While code review is extensively studied for its general challenges, best practices, outcomes, and socio-technical aspects, little attention has been paid to how refactoring is reviewed and what developers prioritize when reviewing refactored code in the Refactor branch. Objective: The goal is to understand the review process for refactoring changes in the Refactor branch and to identify what developers care about when reviewing code in this branch. Method: In this study, we present a quantitative and qualitative examination to understand the main criteria developers use to decide whether to accept or reject refactored code submissions and identify the challenges inherent in this process. Results: Analyzing 2,154 refactoring and non-refactoring reviews across Qt open-source projects, we find that reviews involving refactoring from the Refactor branch take significantly less time to resolve in terms of code review efforts. Additionally, documentation of developer intent is notably sparse within the Refactor branch compared to other branches. Furthermore, through thematic analysis of a substantial sample of refactoring code review discussions, we construct a comprehensive taxonomy consisting of 12 refactoring review criteria.