In D18575#401418, @ngraham wrote:

No other KDE app does this

That's not true. Kate and Gwenview do this too, and for good reason: without this warning, there's no way to know how to get the menu bar back again if you remove it by accident and/or don't know about the ctrl+m shortcut.

Konsole doesn't show a warning because it's a power user app; presumably the user knows what he/she is doing. Dolphin doesn't have this warning because it automatically creates a menu-like toolbar button when the menubar is hidden, which is an acceptable alternative. But for apps that don't do this or something else (e.g. putting the menubar in the titlebar instead) we need to show an ugly warning like this.

I can argue that the ksysguard is also a power user app, but I'll now argue something else:
The hidding / displaying the menus should not be the *app* responsability, but KXmlGuiWindow. If this code is moved there, all apps that are currently doing this manually can benefit the feature and have a tiny bit less code.

In D18575#411906, @tcanabrava wrote:

The hidding / displaying the menus should not be the *app* responsability, but KXmlGuiWindow. If this code is moved there, all apps that are currently doing this manually can benefit the feature and have a tiny bit less code.

Yep, strongly agree.