Improved mouse rectangle selection robustness (bug#38013)
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.