qt5/examples/widgets/mainwindows/mainwindow

Should be reproducible from any platform.  I am using:

$ qmake -v
QMake version 3.1
Using Qt version 5.11.3 in /usr/lib/arm-linux-gnueabihf

$ uname -a
Linux raspberrypi 4.19.118-v7l+ #1311 SMP Mon Apr 27 14:26:42 BST 2020 armv7l GNU/Linux

Found in many versions and seems to be still present in 5.15 (and dev: https://github.com/qt/qtbase/blob/dev/examples/widgets/mainwindows/mainwindow/colorswatch.cpp).   In colorswatch.cpp, void ColorDock::changeSizeHints() the wrong result value is set for accept.  Clicking "OK" does not result in any changes.  Change:

connect(buttonBox, &QDialogButtonBox::accepted, &dialog, &QDialog::reject);

to:

connect(buttonBox, &QDialogButtonBox::accepted, &dialog, &QDialog::accept);

With the update, clicking "OK" allows sizeHint updates.