The key name list defines 'Pause' and 'PrintScreen'. However, GTK defines 'Break' and 'SysReq' defines separately from them. That means that these keys may be different key from 'Pause' and 'PrintScreen'. So, I think that 'Break' and 'SysReq' keys should be defined in the list too.
We've decided that we map GDK_Break to Pause and GDK_Sys_Rq to PrintScreen. That provides better compatibility with Windows.
I'm marking this as INVA for now. If somebody thinks our decision is wrong, feel free to reopen this bug.