In the CodeMix editors, when you select a block for editing (Alt+Shift+A), initial highlighting of the block works, but then after the first button press, the cursor ends up on the last line and repeats the previously pressed character numerous times, and then the block is no longer selected for editing.

In the below screenshot, with TypeScript editor notice the block being selected
https://gyazo.com/a94f121ec0048bcb465a1bc5100e90bd

With CodeMix TypeScript editor, after selecting the block once you start typing, the control moves to the last line and the block is no longer retained. I tried selecting ‘title’ vertically and typing test, which resulted in bad editing as shown below :
https://gyazo.com/81476257b62c5579626ef5151e5bb266