fix the flickering by delaying and throwing

Explanation coming on my blog, math2001.github.io, something like
"lessons learned from re writing MarkdownLivePreview"

live-testing/test.md

@@ -1,4 +1,4 @@
-# hello world  
+# hello world
 
 This is a *test*. Some inline `[2]code()`.
 
@@ -13,6 +13,4 @@ if input("answer yes") != 'yes':
 
 this flickering is really annoying...
 
-this
-
-It looks like it's gone... Oh wait nah, it's still here...
+It looks like it's gone... Oh wait nah, it's still here...