Fixed.

One of the following flags was the solution.
I enabled both.

Print with PostScript Type 42 fonts
opera://flags/#print-with-postscript-type42-fonts

Enables Display Compositor to use a new gpu thread.
opera://flags/#enable-drdc

My fonts are all jaggy. Happens on all websites. Samples below.

No problem with other browsers.

How to fix?

I tried changing my preferred fonts in Opera Settings, but that didn't help.

Thx

some examples

https://addons.opera.com/en/extensions/details/custom-style-script/

with

Others:

https://chrome.google.com/webstore/detail/advanced-page-injector-pl/fmeejpkpickglppdkmoimngnmpkdllao

https://chrome.google.com/webstore/detail/asset-injector-google-chr/obpidgojggdbmochnlgiejlhiepgfmpb

https://chrome.google.com/webstore/detail/my-style/ljdhjpmbnkbengahefamnhmegbdifhlb

I'm able to tweak website CSS in Developer Tools, no problem.

But not with extensions.

I've tried many CSS-based web styling extensions, to add custom CSS to websites. This works fine on Chrome.

I tried eight different styling extensions, including some that i know work on Chrome.

None of them work on Opera GX. I haven't tried regular Opera.

Is there a setting or flag that enables/disables this?

Awful, old-school, not-natural voice. I didn't listen past the default voice.

"Choose File" is Disabled on Bookmark Importer

How to fix?

Solved!
thx

The Tabs menu keyboard shortcut conflicts with an extension.

Is there any way to change or disable the key?

thx

These steps don't work:
https://productforums.google.com/forum/#!topic/chrome/e7fhdYPtWjE

on restart Opera, devtools are still enabled.

Any fix?

Linux 4.15.0-29-generic #31-Ubuntu SMP , x86_64 x86_64 x86_64 GNU/Linux

note, Shift opens in a new window.
Control opens in a new tab.
Any way to make "new tab" the default?