I’m posting a topic because this took me really too long to solve this problem (which should be a non issue to begin with, I’m not really satisfied with that default behaviour and the lack of documentation around it).
So! I wanted to setup <Win>+T to do something specific I’m used to. By default, it opens a terminal window. No big deal, I guess I can easily change that default shortcut from the “Keyboard Shortcut” panel in the Control Center.
First weird thing to know: under Ubuntu MATE, the <Win> key (also often called <Super> on linux systems) is here called Mod4
I don’t find an existing shortcut listed for <Win>+T, so I create a custom one, thinking it will override the default. To my surprise, now <Win>+T triggers my custom shortcut + a terminal window.
Well, after a long research (and help from AI), that’s because there is also another set of shortcut really well hidden, that are handled by Marco, which is MATE Window Manager (a fork of Metacity in fact). I don’t know why these aren’t visible and editable from the control center.
Marco’s keyboard shortcut settings are stored into GSettings (a builtin configuration variable store for gnome applications, it’s a bit like the Microsoft Windows Registry), under the path org.mate.marco.global-keybindings. You can see and edit them using dconf-editor (the graphical tool to access GSettings). There I was able to find a shortcut (without an explicit name) that was set on <mod4>t. I replaced the value with disabled in order to turn it off.
After a long hiatus without interest in open source software, I’ve found the time to dwelve into it again. I hope I can contribute, at least with bug reports and community participation (not sure I'll have the skills and time for programming)