I tested both windowed (my normal way to use the viewer) and real full screen (i.e. without window manager interaction) modes, with the same result: no issue.
I am also using MATE here (although in its last GTK2 incarnation, i.e. MATE v1.14, and with
Sawfish as the window manager instead of Marco). But almost all MATE and Sawfish keyboard shortcuts (with the exception of media-related ones, for the keyboard media keys, and things such as task cycling, which I associated with a shortcuts using the "Windows" key) are disabled on my system... I still believe/feel like your issue stems from one such shortcut in MATE/Marco (and likely in MATE itself, since what changes in full screen mode, is the absence of interaction with the window manager, i.e. likely with Marco in your case)...
The distro itself is likely irrelevant...
No, I do not keep old versions around: no room on web-server and hard disks for them (*). It is the user's responsibility to keep old versions around, just in case...
You still can recompile the current release of the viewer on your system (just a matter of launching the ./build-linux.sh script from the sources) against SDL1 (edit the indra/cmake/00-BuildOptions.cmake to change the USE_SDL2 option to OFF, prior to launching the build script). See the doc/LinuxBuildHowto.txt in the sources for full instructions/help about building the viewer under Linux.
You may also enable the "Advanced" -> "User interface" -> "Debug" -> "Keys" option and watch the debug messages console (SHIFT CTRL 4) to see what are the differences between SDL1 and SDL2 key events...
(*) Keep in mind it has been over 13 years I have been publishing weekly viewer releases, about a third of the time two releases a week (a "stable" one and an "experimental" one), and for both Linux and Windows, with about 80Mb in average for every single binary file (i.e. around 160Mb to 320Mb per week during 13+ years, just for the viewer binaries: over 125Gb of data), plus the viewer sources ("only" 5Gb or so of data, which is half of my web-server quota), and the pre-built libraries binaries tarballs (they do not change every week, but still)... Mind you, my (ISP-based) web-server quota is not infinite (by far) and must be shared between several other websites of mine.