As you may know, our fellow Webkit and Blink contributors from Igalia were busy lately trying to make selection work properly with CSS Regions. Besides dealing with the challenges inherent in a complex code base like WebKit’s and Blink’s, they were also busy updating the test suite for selection in regions. The amount of tests we have now in both engines provides a good starting base for further work on selection in regions, minimizing the chance of code regressions. Moreover, the newly added tests have support for vertical writing modes in addition to the usual horizontal writing modes.
For a more detailed description of their work, check out their blog post! I look forward to seeing more tests submitted, especially related to the performance of selection in regions. If you are interested in this topic, you should follow this WebKit bug.