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 !

Has there been any movement on this. There has been a few updates to Vivaldi and still no mention of this.
It is still on going, and it seems to really happen / trigger upon clicking on links within the page that open a new tab. I noticed that as @LonM mentioned, other tabs are incredibly slow to load some times. I have been tempted so many times to go back to chrome. I really don't want to, but this workflow is killing me.

Any help would be appreciated.

Also @AGoodGeek's workaround didn't work unfortunately. So i have stuck to cloning the tab and closing the old infected one.

Thank you again!

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@mib2berlin

Hi,
I thank you very much for your tip. This solves in great part my problem as my tabs are now represented by a simple text headline. This permits a clearer arrangement with much more tabs per page. The pop up windows in this configuration are still available, when passing over the tabs with the cursor.

In the mean time I was studding an external solution, called 'Tab Outliner', with powerful tree functions. But I prefer the integrated display of tabs for starting to use Vivaldi. Thanks to your help I'm now convinced to leave Firefox as main browser.