OK this seems to be an issue with GTK3. They assume that a Dialog shouldn't have a close button[1][2], The solution is to convert the window type from a Dialog to a Window.
https://github.com/mixxxdj/mixxx/pull/295
[1] https://github.com/nvbn/everpad/issues/88
[2] https://stackoverflow.com/questions/8691025/qt-qdialog-and-gnome-3-how-to-display-the-close-button-title-bar-and-make-it-mo
OK this seems to be an issue with GTK3. They assume that a Dialog shouldn't have a close button[1][2], The solution is to convert the window type from a Dialog to a Window.
https:/ /github. com/mixxxdj/ mixxx/pull/ 295
[1] https:/ /github. com/nvbn/ everpad/ issues/ 88
[2] https:/ /stackoverflow. com/questions/ 8691025/ qt-qdialog- and-gnome- 3-how-to- display- the-close- button- title-bar- and-make- it-mo