[bitwize]

There are two clipboards in X11. One is the middle-click-paste, selection clipboard that gets clobbered when you highlight something. But C-x, C-c, and C-v (in CUA applications) operate on a different clipboard altogether that doesn't exhibit this behavior.

[vbezhenar]

And that is awesome feature that I miss a lot in other operating systems.

[bitwize]

It's a wart of X11, which is obsolete and deprecated. Wayland has a single clipboard which behaves in mostly the standard way.