Here is a picture of the problem:
Notice that all lines of text have horizontal lines similar to underlining. However, this is a plain text editor (Kate) and it does not do underlining. There is no formatting applied to this text.
I selected the text so the lines would show up better in a photo. But the lines exist even without selecting the text. Sometimes they are much thicker and darker. Sometimes they are light. Sometimes they won't be there at all, only to come back at random.
Konsole has the same issue. With white text on a black background, Konsole sometimes show multi-colored horizontal lines. Sometimes every line in Konsole has this ugly and distracting underlining. Sometimes only a portion of the lines have it.
Sometimes the lines are so dense and overwhelming that it is hard to read the text. Other times the lines are mild, as in the attached photograph.
I first saw this issue about a ten months ago on a desktop computer. I thought the user had just done something really crazy in font settings. But now I am seeing the issue on a new laptop without any significant settings changes from default.
Both systems run a fully updated Arch Linux KDE. On this laptop, I reset all font settings (in System Settings) to default values. I also reset the Konsole profile appearance to default settings (even though settings were already at default values).
However, the horizontal lines will not go away.
The applications work correctly (other than sometimes being hard to read text). Copied text does not include the horizontal lines. Commands in Konsole are not affected by the appearance of horizontal lines. It seems to be a display glitch, but it is not specific to any GPU (affects Intel or nvidia) or to any display screen (I tested different monitors on the desktop) or to anything else I can determine.
I tried various fixes on the affected desktop over the last ten months and I have not resolved it on that machine either.
I have multiple other Arch KDE computers that do not have the problem.
Does anyone have a clue as to what might cause this? Has else anyone seen it?
Edit: Please see the KDE bug report for Konsole:
373232 – Horizontal lines with fractional HiDPI scaling