What this says to me is that ARM-64 is not considered stable yet and hence only available in unstable (snapshot) builds. Possibly they just don't have enough testers yet, though you could ask @Ruarí about that. (As far as I know, he's the main guy in charge of Linux builds.)
I have the same issue and I don't want to complain just want to add a little bit of information.
I though situation got better in Beta 3 because although all YouTube videos worked in full screen, the ones that were streamed on youtube live had the same issue and this was fixed in Beta 3. Now i upgraded to the latest snapshot (1.0.430.3) and the windowed full screen on live streamed youtube videos is back (normal and embedded videos works normally). Vimeo, Twitch.tv, TrailerAddict works great as well. I don't have any English website that doesn't work right now but some from my country (Czech Republic) don't - stream.cz (something like czech youtube), csfd.cz (czech version of IMDB) but on the other hand videacesky.cz (non-czech videos with subtitles) works great.
I'm currently using Xubuntu 15.04 and as I said Vivaldi snapshot 1.0.430.3 (or Beta 3 sometimes).
My first comment here so I just wanna say thanks for this great browser, I'm using it as my primary browser. I can always ignore this issue by full screening the window with Alt +F11 as suggested so please focus on bigger issues, this is nothing and once again thanks guys
edit: wrote wrong beta version - Beta 2 -> Beta 3
APT 1.2.7 warning at release files signed with signatures using SHA1.
W: gpgv:/var/lib/apt/lists/repo.vivaldi.com_snapshot_deb_dists_stable_Release.gpg: The repository is insufficiently signed by key ED18652D86E25D422EA7CE132CC26F777B8B44A1 (weak digest)
Blog post from APT-Maintainer Julian Andres Klode
or German article
We will have to update our repositories to meet the new signature requirements coming in Ubuntu 16.04. In the meantime the warning can be ignored since Vivaldi will install normally and update until this is fixed. I have the same warning for many repositories/PPAs right now. But the time 16.04 is release I would assume this is taken care of so that the warnings will disappear.
//Christian - Vivaldi Technologies
I use neither gnome nor kde (or any other desktop environment), and thus not gnome keyring or kdewallet. And I don't really see any use for a login/password manager outside of my browser. However, if vivaldi actually doesn't store passwords as plain text in the "Login Data" database, I'd consider setting up kdewallet (or, preferably, a vivaldi-compatible manager that doesn't pull in a lot of kde or gnome dependencies).
Doesn't work for me!
Fedora 23 64bit.
I used chrome-unstable 50 and flash works fine, I tried to copy the file from
Restarted the browser and the Flash Videos doesn't start.
It didn't give me anywhere to add a string, but when I clicked on a link it asked me what app to use and I told it there.
I did this many years ago when I was getting Firefox and Thunderbird to work together on Win 98. Seeing those settings reminded me of it.