I will copy the thread from Ubuntu Mate 19.10 Beta testing so I can elaborate on it.
As I need a small mobile server for my car/caravan and intended some raspberry 4 for it I just found something interesting on flow market .. an "HP ProDesk 600 G2 Desktop Mini" that looks like in this picture:
As it can also be seen in the picture, this MiniPC has two Display Ports and one VGA.
The issue with it, is the video outputs on current Ubuntu Mate.
If I boot using VGA connection, boot process stall immediately after grub menu.
If I use simultaneously both display ports, I get inconsistent Mate desktop detecting of monitors. Sometime are detected both, and Mate load correctly on both. Sometime either the one or the other monitor are not detected, so it load on that monitor corresponding low resolution, or sometime either the one or the other monitor are not detected, and both monitor are loaded with same low resolution or even same content. If i disconnect the corresponding cable and reconnect it, the monitor is correctly detected. Sometime I just have to go to mate tweak and change resolution back higher.
I don't know why, I feel that this is exclusive related to latest kernel, as it work Ok on previous kernel of a live image of Lubuntu 19.04 used for testing this.
So when I will have time I will download a previous Mate Image and see how it deals, or even try to use an old kernel on current Mate,, I will investigate more also this issue.
I buy another one this week, so on that one I install Ubuntu Mate 18.04, as intended for ZFS stability
Regarding first issue test VGA video output with 5.0 kernel, PC boot Ok, install further 5.2, 5.3 kernels. The issue is specific only to 5.3 kernel tree. On mate 18.04 boot stop on sleek greeter login that is not displayed on 5.3.
Regarding second issue it affect both Mate 18.04 and 19.10, but not Lubuntu 18.04 with LXQT desktop, so this look more like a specific Mate one. But I can detail more
if i remove the cable on the undetected monitor and reconnect relatively immediately (less the same around 5 seconds) issue remain
if i remove the cable on the undetected monitor and reconnect a little bit slower (after 6-7 seconds) monitor will be detected and resolution automatically changed
So maybe this is related to to fast video initialization on Mate maybe.
I also observe inconsistent bios splash on this monitors so I think this is more like hardware/bios related issue that further affect Mate boot wile LXQT desktop maybe after boot is doing some re-initialization or a better one than Mate.
Very Interesting ... on another small PC that I have, an Intel Nuc with a HDMI and VGA out, if I boot with both monitors I get this,.
When I move the mouse, that continue flashing for some time
Some around after 18.04, I get a second mouse pointer somewhere on the border between monitors, the one does not move, but it change shape like the active one, wen show busy.
Right now in 19.04 the other mouse pointer sometime try to move or make jumps near the active one, or some kind like this, so it remain some trailer flashing on screen, like in above picture.
Somewhere in the past, as I don't remember, if power up PC with only one monitor, and move mouse until left border edge, the displayed pointer get stuck couple of pixels away, even the real pointer move accordingly (hidden).
This last issue is a permanent issue from some time. But if I boot with two monitors the issue does not exist, connecting or disconnecting later the monitor, does not change the bad or good behavior obtained on boot.
It looks like the pointer issue on this system gets worse over time, right now no more able to use this mini PC with dual displays, lack that I am using with only one monitor. Very rare I connect a second monitor to this mini PC, like today when I found the issue on 19.10
I am really thinking to test on this one some other distribution, because my feeling is that is also related between Ubuntu Mate kernel interaction or even a latest kernel issue/mesa/xorg....
kernel trash error on boot, and stop mate loading if VGA is connected
VGA is not detect if I connect a cable to it later after boot.
if I install other mainline kernel on 19.10 I don't get ZFS support
So the only option is to downgrade the entire Os (reinstall it) probably 18.04, or something a little bit newer.
I am also thinking to buy this week some adapters like DisplayPort to HDMI, I test one (Display port to DVI with HDMI signal pass through - I know that sound wired, but that is how it work ) and it works, monitor is detected correctly all the time.
So the HDMI output on Display Port on this system looks consistent on Ubuntu Mate, see when I have at least two adapters.
Test 5.4rc4 kernel and VGA port output is fixed back, (just the kernel trash some USB enumerator issues, without seeing any impact on those...) so good news, as sometime an issue might pas couple of kernel revisions.
I will check also, to see if something has improved regarding DisplayPorts monitor detection, (no mood now for display cable swaps).
) and it works, monitor is detected correctly all the time.