This is already possible, for instance:

https://code.onedev.io/projects/160/files/ce8d42208fb1b45c512dd792e849faa1e8eec79e/pom.xml?position=source-11.1-13.28-1

Addendum: could you add option to select only lines / whole lines and option to select them by clicking on line number? Only now I noticed there is a pop-up when selecting source itself.

OneDev always uses mouse to select regions. To select a line, triple click the line. Adding separate logic to select lines will complicate things unnecessarily.

YMMV but not being able to select particular line is quite counterintuitive, but that may be an aftermath of almost all git-related services using it to select line.

Would be nice to have it still :-)