Application resolution vs desktop resolution

Actually I don't think this is a Trinity problem, because reverting to lightdm shows no change, but I post this in hope someone can give me a clue: I have setup a machine based on an Intel based SBC (UP board with a "cherry trail" processor. It runs openSUSE Leap and Trinity. I initially had lots of screen corruption but this I solved by setting the graphics to "xua". My problem is this: while I have the screen resolution setup to 1920x1080 (and trinity desktop looks OK for this resolution), the monitor (which is accessed from HDMI through an HDMI to VGA converter because I use a KVM that needs VGA connectors) was initially "seen" as 1024x768. And the applications seem to retain that resolution. I could make things better in kmail (which is the main application for this machine) by reducing the font size. The other applications still look "too big". So I get the feeling that the lower resolution was saved somewhere... Is there a place where I can tell them to use 1920x768? Have good day, Thierry