I am using Gentoo Linux (no systemd) with XFCE (on X11, no Wayland).

In this environment, Vivaldi never freezes, regardless what you do.

So what you see is not related to Linux, but it is due to an interaction with something else that you use, perhaps Gnome, if that is what you use for the graphic desktop.

You should look at your "Vivaldi Settings: Keyboard", at the key bindings. Perhaps your CTRL+F is bound to something that is wrong for your system.