Handle scrolling when hover popup is open

There are two kinds of scrolling that need to be handled:
1. Scrolling the buffer in a window shifts the buffer, but popups stay
   in place. Instead, we close the popup whenever we receive a
   WinScrolled event.
2. Scrolling the popup contents themselves resets the `updatetime` timer
   and can trigger a second CursorHold event, which leads to YCM
   resetting the hover popup. Instead, only re-display the hover popup
   if it is not already visible.

autoload/youcompleteme.vim

@@ -717,6 +717,7 @@ function! s:EnableAutoHover()
     augroup YcmBufHover
       autocmd! * <buffer>
       autocmd CursorHold <buffer> call s:Hover()
+      autocmd WinScrolled <buffer> call popup_close( s:cursorhold_popup )
     augroup END
   endif
 endfunction
@@ -1621,9 +1622,11 @@ if exists( '*popup_atcursor' )
       return
     endif
 
-    call youcompleteme#GetCommandResponseAsync(
-          \ function( 's:ShowHoverResult' ),
-          \ b:ycm_hover.command )
+    if empty( popup_getpos( s:cursorhold_popup ) )
+      call youcompleteme#GetCommandResponseAsync(
+            \ function( 's:ShowHoverResult' ),
+            \ b:ycm_hover.command )
+    endif
   endfunction