Cell > All Output > Clear needs a popup warning

Nobody has claimed this yet.

It's very helpful that the notebook includes a popup warning if you restart the kernel, informing you that all variables will be lost if you proceed. Since clearing all cell outputs may amount to pretty much the same thing depending on the nature of your code, this option (Cell > All Output > Clear) should have a similar popup warning.

In fact, there is actually *more* need for a popup warning for Clear Output because the menu structure makes it easy to do accidentally (not true for kernel restarts). In the Cell dropdown menu, if you move your mouse diagonally from the "Current Output" to the "Clear" option in the ensuing submenu, you may inadvertently mouseover and trigger the "All Output" submenu instead. This menu is precisely identical to the "Current Output" submenu, so it's easy not to notice. Then clicking the same "Clear" option suddenly and irrevocably clears your entire notebook.

I blame no one but myself for the seven hours of runtime I lost to said click -- but upon reflection it seems really, really easy to do this. I'm sure I'm not the first. Given how simple it would be to throw in a popup warning for this, I think it would be a helpful and sensible addition.

Start from the Cell > All Output > Clear menu action and compare it with the existing kernel-restart warning described in the issue. Done means attempting to clear all cell outputs shows a confirmation popup before the irreversible action, including the risk of losing useful runtime results.

Written by the indexing model from the issue text.