Add confirmation dialog when leaving the webui

[leszekhanusz]

New feature request

I was recently writing a long story in the notebook section of the webui when, by mistake, I clicked on a bookmark in my bookmark toolbar instead of clicking the Parameters tab of the webui.

So I left the page. I tried to use the back arrow of my browser to go back to the webui but all my output was gone...

It should be possible with javascript to ask the user if he really wants to exit using the beforeunload event as shown here.

This would prevent any unsaved output to be lost.