V0.72
The applet now works on and can be built for both GTK2 and GTK3 versions of MATE.
Aside from the changes to layout containers (i.e. GTK2 VBox & HBoxes become GTK3 Boxes/Grids), underlying differences between the two toolkits meant that the code to calculate window minimise positions and window list popup positions has had to be reworked.
A GTK2 version of the applet can be produced by running ./configure --prefix=/usr
during the build process. If a GTK3 version is required, running ./configure --prefix=/usr --with-gtk3
will do the trick.