Try turning off
Preferences > Plug-Ins > Allow Extensions …
Preferences > Plug-Ins > Load Extension Panels
and restarting Photoshop.
thank you very much, problem solved!
Thanks to both of you for the help, I tried the first simply because I tried in order - but assuming they both may work - any down sides to either or is one method better than the other for overall performance?
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Disabling GPU usage invalidates GPU dependent features (for example the Rotate View Tool, …), disabling the extensions-related settings would for example disable the Library Panel.
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We are happy to announce that the PNG distortion issue should be fixed with the Photoshop CC 2018 (19.1.2) update which is now available.
Please install the update and let us know if you still see this issue.
To update, click Update next to Photoshop CC in the Creative Cloud desktop application.
Just in case, If you don't see the update right away, click the 3 dots in the upper right and choose "Check for App Updates". See Available updates not listed for Adobe Creative Cloud applications