Vivaldi has poor support for Chrome extension features, including typing tb<spacebar>. Vivaldi doesn't have the Chrome omnibox, so this feature doesn't work. This was reported elsewhere, and the Vivaldi team doesn't plan to fix this.
Regarding the other issue, that also happens occasionally (not always), and only on Linux, and we are not sure why it happens.
If we find any other information, we'll keep the thread updated.