I've been using KDE for quite a while, and this one has me stumped. I'm running frameworks 5.64.0 and plasma 5.17.4 on Gentoo linux (openrc). I'm not actually sure when this started, but it's relatively recent. Video card is [AMD/ATI] Cape Verde XT [Radeon HD 7770/8760 / R7 250X] driving two monitors at 1920x1080 each by DVI. The primary monitor is on the right, configured as desktop. The monitor on the left is configured as folder, showing ~/Desktop. Background of each monitor is the "Next" image.

At the upper left of the right monitor is an area about 240x139 pixels (estimated by capturing a rectangle with Spectacle) which I don't quite know how to best describe. It appears several shades lighter than the background image - or perhaps a white partially transparent box. I cannot select it or click on it (right or left click). It does not show up in a spectacle capture. It stops exactly at the left edge of the right monitor, and does not extend over the right edge of the left monitor. I've looked in ~/.config/plasma-org.kde.plasma.desktop-appletsrc, but don't see anything with height or width anywhere close to the dimensions of the mystery rectangle. (I became aware of that file while hunting down a non-displayed panel which had a widget which had messed up all my application menus.) It is NOT an activities widget or a desktop toolbox.

While I'd love for someone to be able to tell me what it is and how to get rid of it, I'll be happy with any ideas what other info to post, or how to further troubleshoot myself, since I know I haven't really given enough information.

Thanks for any suggestions.

Edit 20 Mar 2020: marked solved.

Maybe a screenshot of that area could be helpful?

What tool would you suggest? As I said, I tried Spectacle, and the "artifact" does not show up in the capture. Next step would be to use a real camera or phone, and I'll certainly consider that.

I suggest using a phone, as other screen capture programs will probably fail as well. We really only need a view of the specific section showing what the problem is.

The first shows the corners of both monitors (shelf support between them) although the issue is fairly faint. You can see the right edge of the problem, but not the bottom, which is just below the top of the konsole window. You may need to click the image to see the original. It's not much larger, but a bit clearer.

This is just the main monitor, with the issue a bit more visible. You can also see that the effect is also visible on the top of the konsole window itself.

This is with the background changed to solid color. With a dark color, the problem is even harder to see - as if it just lightens the color a small amount. The graininess seems to be an artifact of the capture.

This is using a webcam. I'll try later with other devices to see if I can actually get a better display of the problem.

Edit: adding a larger image from phone, should show the problem better.

This very much looks like a compositing artifact, especially in the lowest image.

Does disabling compositing help?

Turning off "Enable compositor on startup" under Compositor in System Settings has no apparent effect, unless "on startup" matters. I won't have a change to restart plasma for a bit but I'll let you know if it changes anything. Let me know if I missed someplace else to set this.

Turning off compositing makes no difference, even with a restart of X/Plasma.

New observation: the mouse icon is NOT affected by this - it doesn't change appearance at all when entering this area. That seems consistent with your thought that this relates to compositing, except that the problem still occurs with compositing turned off. I can try upgrading to plasma 5.18, but that will take a bit of effort, so I only want to try if it's likely to make any difference.

I was wondering: if you close all open applications in all Desktops, does the rectangle persist?

Yes. Kill all apps, turn off compositing, exit and restart plasma - rectangle still present on all desktops.

I don't know if this is relevant, but if I hit the "Show Desktop" key - configured in Settings->Shortcuts->Global shortcuts0->Activity Switching->Show Desktop, a small rectangle like that appears in my monitor corner when the open windows are minimized.

Section_8 wrote:I don't know if this is relevant, but if I hit the "Show Desktop" key - configured in Settings->Shortcuts->Global shortcuts0->Activity Switching->Show Desktop, a small rectangle like that appears in my monitor corner when the open windows are minimized.

which is not a persistent one, as it changes depending on the open applications, and this is clearly a compositing artifact. I don't think this is the same issue here.

For unrelated reasons, I just restarted my desktop with Wayland instead of X, and the artifact is still present. Does that give any better hint as to where the problem might be? I can find the Systemsettings page for compositing, but I don't see any way of turning it off. Am I missing something, or is that a "feature" of wayland?

Edit: I'm currently on Plasma 5.17.5 and Frameworks 5.67.1

So that rectangle is always there, regardless if you choose X or Wayland?
Maybe you are using a background picture that actually has this rectangle by default, did you try changing the desktop background?

If this persists across different background images, then the only thing I can think of is a hardware problem with your monitor.