Re: Issue with rectangle mouse selection

["Mattias Engdegård" <mattiase@acm.org>]

> Terminal.app and Gnome Terminal are fine, but Xterm exhibits the behaviour you describe.

XTerm does indeed not report mouse movements when modifiers are used, so there is little Emacs can do about it.
This may be intended; the source (button.c) contains

    /*
     * We reserve shift modifier for cut/paste operations.  In principle we
     * can pass through control and meta modifiers, but in practice, the
     * popup menu uses control, and the window manager is likely to use meta,
     * so those events are not delivered to SendMousePosition.
     */

If this bothers you, perhaps you could ask the maintainer to reconsider; gnome-terminal manages, after all. (XTerm clearly needs more options. Just like Emacs.)