GitLab

Make the rectangular selection work better with
display-line-numbers-mode and side-by-side windows.
Also make the mouse track the text cursor in a consistent way.

* lisp/mouse.el (mouse--rectangle-track-cursor): Added constant.
(mouse-drag-region-rectangle): Take the line-number width into
account, and use window-relative columns.  Track either the cursor or
rectangle corner with more care.