Fix scrolling glitch during preview

Typing while in preview mode would occasionally lead to a scrolling
glitch, where scroll would briefly be at 0, before jumping to the
location it was supposed to be in in the first case.

This happened due to the async nature of JS calls, in the following
scenario:
1. Load
2. Read started
3. Read finished
4. Read started
5. Load
6. Read finished

The results from op 4 would be invalid due to loading in-between, and
handling the result in 6 would set the wrong scroll value.

This change ensures results are discarded whenever we are waiting for
them and a new load starts.

uberwriter/web_view.py

@@ -42,6 +42,7 @@ e.scrollTop = (e.scrollHeight - e.clientHeight) * scale;
 
         self.state_loaded = False
         self.state_load_failed = False
+        self.state_discard_result = False
         self.state_waiting = False
 
         self.timeout_id = None
@@ -56,6 +57,7 @@ e.scrollTop = (e.scrollHeight - e.clientHeight) * scale;
     def on_load_changed(self, _web_view, event):
         self.state_loaded = event >= WebKit2.LoadEvent.COMMITTED and not self.state_load_failed
         self.state_load_failed = False
+        self.state_discard_result = event == WebKit2.LoadEvent.STARTED and self.state_waiting
         self.pending_scroll_scale = self.scroll_scale
         self.state_loop()
 
@@ -74,16 +76,20 @@ e.scrollTop = (e.scrollHeight - e.clientHeight) * scale;
     def read_scroll_scale(self):
         self.state_waiting = True
         self.run_javascript(
-            self.GET_SCROLL_SCALE_JS, None, self.sync_scroll_scale)
+            self.GET_SCROLL_SCALE_JS, None, self.finish_read_scroll_scale)
 
     def write_scroll_scale(self, scroll_scale):
         self.run_javascript(
             self.SET_SCROLL_SCALE_JS.format(scroll_scale), None, None)
 
-    def sync_scroll_scale(self, _web_view, result):
+    def finish_read_scroll_scale(self, _web_view, result):
         self.state_waiting = False
-        result = self.run_javascript_finish(result)
-        self.state_loop(result.get_js_value().to_double())
+        if not self.state_discard_result:
+            result = self.run_javascript_finish(result)
+            self.state_loop(result.get_js_value().to_double())
+        else:
+            self.state_discard_result = False
+            self.state_loop()
 
     def state_loop(self, scroll_scale=None, delay=16):  # 16ms ~ 60hz
         # Remove any pending callbacks