The "pause" happens, because "alt" is a system hotkey that sets the focus to the window's system menu (restore, minimize, maximize, exit, etc.) This menu exists if the window has a caption bar (it is the one you get if you left click on the icon in the title bar, or rightclick the title bar). So the quickest way to prevent this behavior without python or some plugin is to disable "caption" in Application Properties/Window Properties, or to go fullscreen.
To be honest, this is the first time that I see this behavior. It is an issue that should be avoided by Construct if possible.