Lubuntu 20.04 qa-test install; flashing at sddm greeter when mouse at top panel (pre-login)

First boot after new install; Lubuntu 20.04 QA-test install
dell [optiplex] 755 (c2d-e6850, 5gb, amd/ati radeon rv516/x1300/x1550)

NOTE: I re-used partitions, /dev/sda8 (/) was empty, but /dev/sda9 (/home) contained data. sddm wasn't used as DM before Lubuntu 20.04 installs, so I doubt it's related to a conf/config issue relating to sddm.

Description:

At first boot, I expect the cursor to move around the display without bright flahes of WHITE covering parts of the screen; that are very bright/distracting (somewhat strobe like).

It only has this effect when the cursor/pointer is moved at the top of the display in what I'd describe as a panel. If the cursor is over the 'top' bar I get a strobbing effect as the cursor/pointer moves. Stop moving the mouse and the effect stops, move it off the top 'panel' and it's gone too.

The 'flash' or 'strobe' like effects covers the whole screen, and occurs when the mouse is over
- Session drop down box (not word itself, the Lubuntu drop-down bit)
- Layout - US flag drop down
- Pause/Reboot/Off icons top right.
Elsewhere on the top 'panel' and there is no effect.

It also occurs at the left arrow near the "Chris Guiver" password area, no issue if I move over white-grey-person-password area, just over the "<" to the left of that box. This could be a real possibly should a person react to 'flashes' (I'm thinking epilepsy but just a thought).

The screen appears to flash all WHITE with a triangle pushing out from the left (reminds me of screen tearing). I haven't noticed this before on other boxes.

On logging out, I get a brief strobe too before or as `ssdm` is drawn.

This could be filed again wrong package, I suspect it relates to xserver-xorg-video-radeon or something GPU related.

`sudo lshw -C video` reports

guiverc@d755-5-2004:~ $ sudo lshw -C video
[sudo] password for guiverc:
  *-display:0
       description: VGA compatible controller
       product: RV516 [Radeon X1300/X1550 Series]
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0
       bus info: pci@0000:01:00.0
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress msi vga_controller bus_master cap_list rom
       configuration: driver=radeon latency=0
       resources: irq:16 memory:d0000000-dfffffff memory:fe9e0000-fe9effff ioport:dc00(size=256) memory:c0000-dffff
  *-display:1 UNCLAIMED
       description: Display controller
       product: RV516 [Radeon X1300/X1550 Series] (Secondary)
       vendor: Advanced Micro Devices, Inc. [AMD/ATI]
       physical id: 0.1
       bus info: pci@0000:01:00.1
       version: 00
       width: 64 bits
       clock: 33MHz
       capabilities: pm pciexpress bus_master cap_list
       configuration: latency=0
       resources: memory:fe9f0000-fe9fffff

I did NOT expect two cards to be recognized?

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: sddm 0.18.1-1ubuntu2
ProcVersionSignature: Ubuntu 5.4.0-14.17-generic 5.4.18
Uname: Linux 5.4.0-14-generic x86_64
ApportVersion: 2.20.11-0ubuntu18
Architecture: amd64
CurrentDesktop: LXQt
Date: Tue Mar 3 18:48:23 2020
InstallationDate: Installed on 2020-03-03 (0 days ago)
InstallationMedia: Lubuntu 20.04 LTS "Focal Fossa" - Alpha amd64 (20200302)
SourcePackage: sddm
UpgradeStatus: No upgrade log present (probably fresh install)