I experience a similar issue (if not the same one) under windows 8.1 (mIRC 7.41). I don't know if it's a Win8.1/10 specific issue or not because I haven't checked this version of mIRC on my Win7 machine, but I did not experience the issue with previous versions of mIRC in Windows 7.
//while (1) /
Typing the above sends mIRC into a breakable loop (as expected).
Pressing ctrl+break immediately will break out of the loop (as expected).
However, if I wait until mIRC is shown as unresponsive then ctrl+break will NOT work. In previous versions of Windows and mIRC this was not the case for me.
I have found that mIRC will respond again to ctrl+break during this "(Not Responding)" state if any other window is made active. So clicking on the task bar, your web browser, the desktop, etc. will cause ctrl+break to work again as long as you don't click back to mIRC first.
