@Gwen-Dragon Reinstalling fonts had helped. I have apt purge fonts-noto-color-emoji and then installed them again.
Then all instances of Vivaldi widows have been closed.
Thank you for help 🙂

@Gwen-Dragon Here VB-67740 . Did IT 💪

@Gwen-Dragon Ok, so I reinstall helvetica font and now it work well 🙂 Maybe previous font on system was broken.
Thx for help.

@raed @DiVisici thanks a mil for sharing all these helpful information. I have a few more questions: 🤓

Does this issue happened in any other Chromium browsers as well?

Are you able to reproduce this issue on a standalone installation of Vivaldi?
We explain how to install a standalone version here: https://help.vivaldi.com/article/standalone-version-of-vivaldi/
Sharing this insight will help us to understand if the issue is connected to your profile or not.

Lastly please visit vivaldi://gpu and copy paste the report to me in PM so we can see if it's a hardware issue.

Thanks a mil in advance for your help 🙂

Merci à tous pour vos réponses. Mon problème a été résolu... En remettant mon PC à neuf. Je vous rassure, pas à cause de Vivaldi mais à cause de la dernière mise à jour Windows qui a fait planter mon PC.
Depuis que j'ai tout réinitialisé, je n'ai plus ce problème d'affichage, mais ça restera un mystère !

Merci à tous pour votre aide tout de même !

Thanks for help, have a nice day 🙂

FYI, there are a few other (undocumented and unsupported) ways to fine-tune font rendering on macOS:

How to Fix Blurry Fonts in MacOS Mojave for Non-Retina Displays
http://osxdaily.com/2018/09/26/fix-blurry-thin-fonts-text-macos-mojave/

My fellow above and me have a
Open the terminal and issue the following command:

/usr/bin/vivaldi-snapshot --user-data-dir="/tmp/VIV-TST" &

It will not interfere in your Vivaldi system wide install. Same as a first run.
In case there is no warning, then the issue lies on your profile.

@BoneTone Correct.
If placed on the shortcut works only from there; if placed as flags is bind to the Vivaldi installation. It's matter of preference.
--flag-switches-begin [...] --flag-switches-end could be omitted: is a place-holder to recognize which command lines are add from users and which from vivaldi itself.
I don't see --silent-debugger-extension-api [Link] from flags. But usually debug-packed-apps [link] is sufficient.