These use the Qt C++ API's, as well as QML code, supported through the Quick module. QML supports a subset of JavaScript features, and it allows you to develop user interfaces quickly, and provides a convenient way of accessing low-level calls.
The third and last screenshots are of programs compiled the older and stable Widgets API's, ideal for desktop applications.
The great aspect of Qt's libraries is that they are available for different compilers and targets, meaning that I have been able to test my programs on GNU/Linux and Microsoft Windows.
Program names and Git repository URLs for the screenshot number, corresponding to the order in which they appear:
1: explorer, https://github.com/ben-cottrell-nz/explorer/
3: paintbrush, https://github.com/ben-cottrell-nz/paintbrush/
4: elemental, https://github.com/ben-cottrell-nz/elemental/
5: audioviz, https://github.com/ben-cottrell-nz/audioviz