Thanks for your bug report. I was able to reproduce this issue under Windows 10.
As far as I can tell, this is due to a change in how Windows 10 handles applications that it thinks are frozen. If an application looks frozen, Windows 10 blocks any direct input to it. However, if you switch to another application and type Control+Break, mIRC is able to see it as the Control+Break key combination is processed by mIRC regardless of which application it is typed into.
I have found a solution that makes Windows think that mIRC is not frozen during a script loop (by making mIRC peek window messages without removing them) and this prevents Windows from marking the application as "not responding" and/or blocking input. However, this method also prevents you from Alt+Tabbing to mIRC during the script loop. The reason is that Windows assumes that mIRC is not frozen and can therefore display its own GUI. When Windows thinks an application is frozen, it displays a copy of the GUI when you Alt+Tab to it.
That said, using this new method makes more sense as it correctly lets Windows know that mIRC is not actually frozen and is currently processing a script loop. This change will be in the next version.
