However, this is not the right, or at least the complete, fix, because
it has some bad side-effects: after disabling t-m-m and then selecting a
rectangle with the mouse, now typing `M-x transient-mark-mode' still
says it is disabled; after typing it again, it is now enabled, but
typing C-SPC and then moving point does not show an active region; only
after typing C-SPC C-SPC and then anything (e.g. C-g) and then again
typing C-SPC and moving point is an active region shown.  So a better or
more complete fix is needed.

In GNU Emacs 27.0.50 (build 26, x86_64-pc-linux-gnu, GTK+ Version 3.24.5, cairo version 1.16.0)
 of 2019-11-29 built on strobe-lfs84
Repository revision: b2790db049da98b541d07bac21ca7d7c220d3be0
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Linux From Scratch

Configured using:
 'configure --with-cairo 'CFLAGS=-Og -g3'
 PKG_CONFIG_PATH=/opt/qt5/lib/pkgconfig'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS JSON PDUMPER LCMS2
GMP

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix