Qt ("cute") is a cross-platform user interface toolkit available both in both a free GPL edition and a commercially licensed edition. It is used by a lot of free software including the KDE Plasma and the LXQt desktop environments.
The Qt toolkit is free software available under the GNU GPL. The Qt Company, who makes Qt and controls the source code, is also selling Qt as a commercial product under a commercial license. They have signaled that they may start restricting all Qt releases to paying customers for 12 months after new releases are made.
High DPI Scaling
Qt 4.15+ supports automatic high DPI scaling. This feature can be enabled or disabled with a environment variable called
QT_AUTO_SCREEN_SCALE_FACTOR which is set by some desktop environments but not others. Some applications will behave as if
QT_AUTO_SCREEN_SCALE_FACTOR=1 while others will behave as if it is set to
QT_AUTO_SCREEN_SCALE_FACTOR=0 if that variable is unset.
Random Qt and KDE applications blowing up and becoming too gigantic, or being too small depending on your preferences, can be solved by setting
QT_AUTO_SCREEN_SCALE_FACTOR to either
Qt tries to get the scale factor either set or detected by the system it is running on (see HOWTO set DPI in Xorg).
QT_SCALE_FACTOR can be used as an alternative to
QT_AUTO_SCREEN_SCALE_FACTOR. Do note that automatic scaling will override the
QT_SCALE_FACTOR variable if
QT_SCALE_FACTOR can be set to any numeric value such as
Qt 5 scaling is documented on doc.qt.io/qt-5/highdpi.html.