
Hi @radoslavius
I can reproduce the issue in Edge sidebar. As far as I know, there's no way to open page on new tab with JavaScript in Edge sidebar. You can only right click on the link and choose "Open link in new tab" or click the icon on the top to open the page on new tab.
I suggest that you can provide feedback to Edge team by pressing Alt+Shift+I in Edge to request add the feature you want. They'll check the feedbacks and improve the product continuously.
For your another question, it's by design that Edge sidebar by default loading a mobile version. For some not responsive sites, loading a desktop version on sidebar will cause the UI out-of-order display. You can't force the page to open in desktop mode in sidebar.
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Regards,
Yu Zhou