fix scrolling on slate inside scrollable container that is not a window (#1369)

packages/slate-react/src/utils/scroll-to-selection.js

@@ -23,7 +23,7 @@ const OVERFLOWS = [
 function findScrollContainer(el) {
   const window = getWindow(el)
   let parent = el.parentNode
-  let scroller = window
+  let scroller
 
   while (!scroller) {
     if (!parent.parentNode) break
@@ -38,6 +38,8 @@ function findScrollContainer(el) {
     parent = parent.parentNode
   }
 
+  if (!scroller) return window
+
   return scroller
 }