Popup should not go offscreen on resize

[lapo-luchini]

When screen is resized (it happens often to me because I have a popup with a text field and android opening the keyboard resizes the full-screen-map) the center of the map is used as pivot, but if there is a popup on screen it would be nice to keep the popup visible (just like the popup does when it opens up in the first time).

I tried doing that with commit lapo-luchini/Leaflet@6940786 but that doesn't really work if the popup doesn't fit on screen (his own panBy generates a new moveend and it keeps bouncing left and right if wider than the screen). Proper fix would probably need firing viewreset after a resize, but I didn't find a nice way to do that (should be done on animation end, but that's managed elsewhere and it's not easy to modify).

[mourner]

Done. #1308