There's an oddity when using shift and the left arrow key to select text, where it will skip the first character and then start over. This only happens when using the left arrow, and only at the last word in a line, which makes me think that there's something strange going on with how whatever this is treats the ends of lines.
Highlighting using arrow keys top to bottom and right to left seems fine. But highlighting left to right and bottom to top, seems to cut off highlighting and it doesn't highlight the text as you expect, but instead un-highlights sections you've previously highlighted.
Reproducible here https://demos.telerik.com/aspnet-ajax/editor/examples/overview/defaultcs.aspx
Any help would be appreciated.