It seems that suddenly after a restart of the machine I have developed display problems with text when running X. At first the screen was very magnified perhaps 4x then after fiddling around with things, the fonts became normal but rather pixelated. The taskbar panel in KDE5 has disappeared. Reinstalling KDE5 doesnt help. The view of the fonts is the same in xfce and fluxbox but in terminal modes everything looks fine. pictures and graphics are normal and crisp. I have uninstalled and reinstalled Nvidia and tried Nouveau but the results are the same.

I am running slackware-64 -current and it is running the latest updates to current. Other operating systems on the machine on different disk drives are ok. Any ideas what to look for?

This is just a rather wild guess but every time I have experienced problems with fonts, editing "/etc/X11/xorg.conf" to add...

.. iirc, under the "Device" section, has solved it. I can't know if this will work for you but it is likely worth a shot

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@enorbet:
I tried what you suggested and it did not seem to help.

Fonts still scraggly in xfce and kde5 and no task manager panel in kde5
Everything otherwise works ok albeit I only have the right mouse click menu in kde for running things.

I got a fix by the following...

slackpkg reinstall x
reboot

Then I reset the anti-aliasing range which was set to a strange value.
and I restored the panel in KDE5 via the right-click popup menu. I left enorbet's line in xorg.conf as he suggested. Things are reasonably back to normal but I am a bit unaware of what precipitated the scrozzling of X stuff.

The "UseEdidDpi False" line simply protects against some monitors, especially modern TVs that double as PC Monitors, with dodgy EDID code. It isn't hard to toggle to check if yours is or isn't. I'm glad you got it working OK but the emboldened line is exactly why I don't even try automated managers like Slackpkg. I don't wish to cast any negativity towards that specific application but I need to know exactly what is going on or I get lost and befuddled. Slackware on it's own requires extremely little maintenance when we keep it simple.

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Further discoveries on the probable cause: Unplugging and plugging in different video displays.
My machine is a DIY luggable with a M-ITX AsRock mainboard and a 1000W GreatWall Power supply and a Gigabyte MITX style Nvidia 1060 video card. I take this machine travelling with me around Asia and I plug in the video card to a wide range of display devices. The NVidia/Gigabyte card has two DVI-D ports, a HDMI port and a Display Port adapter. On the road in meetings I plug in my Wacom pad and what ever type of video display the meeting room has. I have various adapters hdmi to vga, dvid to vga, dvid to hdmi, display port to vga. Most meeting rooms use either HDMI or VGA setups and most new ones are 1980x1280 which is no sweat at all. I can drive up to four monitors off the videocard without screentearing or lockup, but seldom use more than 3 and usually only one or two monitors. The wacom pad uses HDMI input and i like it on the lowest numbered display port, usually with a small DVI-D to HDMI adapter. I also use this monitor at home and at the office/lab with dual monitors, usually with the wacom pad.
My recent problem started recently at a reboot after unplugging and plugging some displays. I got weird magnification of the screen, scrozzled fonts, and a disappeared panel in KDE. I got things back to normal after reinstalling X and retweaking some menu items. But where did that taskbar panel go? Today I plugged in a hirez monitor to assist in some multiscreen work and lo and behold there is the lost taskbar panel at the bottom of the added screen! and the rebuilt one on the other screen!
So I think I need to study a lot more about how X works and how it interacts with added and removed monitors in ways that are beyond the reach of the Nvidia and KDE settings to adjust and configure. Just thought you all would like to know.

You might benefit from a little study on the "xrandr" and "cvt" commands. They can help a great deal when you resize a lot on different displays. I've recently been fighting some monitor issues myself and they have helped me greatly. Now if I could only see the full width of boot text pre X on some cranky monitors.....

I used to use cvt and xrandr frequently but havent thought that I needed to recently because it seemed that the video card figures it out these days. I used to have some little shell scripts to set up each monitor but wow that's been a while back. had sort of forgotten about doing that.