Viber on Linux, in particular, Ubuntu 18.10
Viber did not update its Linux port for a while, so by now there are two issues that cause trouble with it, even though it's proclaimed to work on Ubuntu.
1. The infamous libcurl3 issue.
That's a really stupid problem caused by dependency on libcurl3 in viber.deb, even though libcurl4 works just fine. Since those two are mutually exclusive, installing anything that requires libcurl3 will remove Viber, and installing Viber will remove libcurl4 and everything that depends on it.
The simplest solution is even more stupid, use RPM intended for Fedora, and convert it to Debian package using Alien: As root:
alien --to-deb viber.rpm
The result is perfectly installable.
2. Blank text issue.
That happened recently to me on a box with Intel graphics. Apparently some upgrade messes up text if old shader cache is present in .cache/qtshadercache , so all text disappears while all layout is fine. Just delete that directory:
rm -rf ~/.cache/qtshadercache
It will be re-created, and font rendering will work again.