SOLVED!!! By a suggestion from the guy on the Intel forum.
The trick is to disable "Panel Self Refresh" in the Power settings of the Intel Graphics Control Panel. I had tried that, but it turns out it only takes effect after rebooting (which it doesn't say).
The hours I spent on this... Whoever thought that enabling 'panel self refresh' was a good idea? I guess this can be added to the list "thing you should immediately disable after buying an XPS", along with the incredible Adaptive Brightness Control (ABC) in the BIOS.
Alan, thanks for your time. Both issues are now solved: the lines in the color bars are just a weird software glitch in SupportAssist, and the artifacts disappear when disabling 'Panel Self Refresh'. However, I think a lot of time and frustration for other users can be saved if this is somehow let known to them, or if 'Panel Self Refresh' is disabled by default for the next driver update. (I am still curious what the possible advantages of panel self refresh are.)
I did a bit of googling, and found out what the function of 'Panel Self Refresh' is. It is a power-saving feature, which uses a kind of extra cache memory for the case where there are static elements on the screen, then the data is sent from the cache memory instead of the main graphics memory (if I understand correctly) which saves power. Apparently this does not work well on our laptops. So we are now sacrificing battery life to circumvent this issue. The question remains:
Does this occur only on our (mine, and the user on the Intel forum LG000) laptops (if exchanged under warranty, does the problem disappear?) or on all XPS 9380 FHD 13's? Maybe also on the 15-inch version? Maybe on ALL laptops using Intel UHD 620 graphics? Maybe on ALL laptops using any Intel graphics with Panel Self Refresh?
I would like to know if the problem occurs on other laptops of the same type. If not, I might want to use my warranty to get a new one.
Regarding the vertical lines in the color bars... I have found out this lines are actually there in the PNG file I got from the screen capture. So it just seems the test pattern has these vertical lines in them, maybe SupportAssist uses a compressed image. So this issue can be ignored, and should definitely not be a reason to swap my LCD screen!
You might wonder why I didn't see the vertical lines on the external TV. Turns out they are actually there, just very faintly.
I'm glad to hear you got the issue resolved by disabling the panel self refresh. I'm surprised that feature is now showing up as it was previously removed via a combination of a BIOS update and an update to the graphics driver as it was causing issues with screen blanking and flickering.
I believe this was connected to any system running the Intel HD620 graphics driver. I'll pass this information back to engineering.
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