Scroll views up if needed to show all their content

There are many situations where this can arise. Some examples are:
- the terminal window is small, and you are showing a view that shows more
  content than fits into the view port, and the view is scrolled all the way
  down; now you resize the terminal window to a taller size. Previously, the
  scroll position of the view would stay the same, so it would add blank space
  at the bottom; now it will scroll to fill that blank space with content
- expandFocusedSidePanel is on, you go to the bottom of a list view, now switch
  to a different panel, then scroll that (now unfocused) panel all the way down
  with the scroll wheel; now you focus that panel again. It becomes larger
  because of the accordion behavior, but would show blank space at the bottom.

And probably others that I can't remember right now. I only remember that I
always found it confusing to look at a view that had blank space at the bottom
even though it had more content to scroll into view.

pkg/gui/layout.go

@@ -73,6 +73,19 @@ func (gui *Gui) layout(g *gocui.Gui) error {
 		}
 
 		mustRerender := false
+		newHeight := dimensionsObj.Y1 - dimensionsObj.Y0 + 2*frameOffset
+		maxOriginY := context.TotalContentHeight()
+		if !view.CanScrollPastBottom {
+			maxOriginY -= newHeight - 1
+		}
+		if oldOriginY := view.OriginY(); oldOriginY > maxOriginY {
+			view.ScrollUp(oldOriginY - maxOriginY)
+			// the view might not have scrolled actually (if it was at the limit
+			// already), so we need to check if it did
+			if oldOriginY != view.OriginY() && context.NeedsRerenderOnHeightChange() {
+				mustRerender = true
+			}
+		}
 		if context.NeedsRerenderOnWidthChange() == types.NEEDS_RERENDER_ON_WIDTH_CHANGE_WHEN_WIDTH_CHANGES {
 			// view.Width() returns the width -1 for some reason
 			oldWidth := view.Width() + 1