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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.

"Free software"

qBittorrent v4.3.0alpha1 seeding Fedora Linux's Xfce spin which is available as a torrent at torrents.fedoraproject.org. qBittorrent is written using the Qt toolkit.

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 0 or 1.

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_AUTO_SCREEN_SCALE_FACTOR=1.

QT_SCALE_FACTOR can be set to any numeric value such as 1.1, 1.3.

Qt 5 scaling is documented on doc.qt.io/qt-5/highdpi.html.

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