Fix scroll jump when previewing documents with math

MathJax loads asynchronously and can alter the height of the document.
Altering the height alters the scroll.

Ensure MathJax is either unused or finished loading before reading
scroll from the preview.

uberwriter/web_view.py

@@ -14,16 +14,37 @@ class WebView(WebKit2.WebView):
     Reference: https://github.com/aperezdc/webkit2gtk-python-webextension-example
     """
 
-    GET_SCROLL_SCALE_JS = """
-e = document.documentElement;
-e.scrollHeight > e.clientHeight ? e.scrollTop / (e.scrollHeight - e.clientHeight) : -1;
-"""
-
-    SET_SCROLL_SCALE_JS = """
+    SYNC_SCROLL_SCALE_JS = """
 scale = {:.16f};
+write = {};
+
+// Configure MathJax.
+if (typeof hasMathJax === "undefined") {{
+    hasMathJax = typeof MathJax !== "undefined";
+    if (hasMathJax) {{
+        MathJax.Hub.Config({{ messageStyle: "none" }});
+    }}
+}} 
+
+// Figure out if scrollable and rendered.
 e = document.documentElement;
-e.scrollTop = (e.scrollHeight - e.clientHeight) * scale;
-"""
+canScroll = e.scrollHeight > e.clientHeight;
+wasRendered = typeof isRendered !== "undefined" && isRendered;
+isRendered = wasRendered ||
+        hasMathJax && MathJax.Hub.queue.running == 0 && MathJax.Hub.queue.pending == 0;
+
+// Write the current scroll if instructed or if it was just rendered.
+if (canScroll && (write || isRendered && !wasRendered)) {{
+    e.scrollTop = (e.scrollHeight - e.clientHeight) * scale;
+}}
+
+// Return the current scroll if scrollable and rendered, or -1.
+if (canScroll && isRendered) {{
+    e.scrollTop / (e.scrollHeight - e.clientHeight);
+}} else {{
+    -1;
+}}
+""".strip()
 
     __gsignals__ = {
         "scroll-scale-changed": (GObject.SIGNAL_RUN_LAST, None, (float,)),
@@ -38,11 +59,11 @@ e.scrollTop = (e.scrollHeight - e.clientHeight) * scale;
         self.connect("destroy", self.on_destroy)
 
         self.scroll_scale = 0.0
-        self.pending_scroll_scale = None
 
         self.state_loaded = False
         self.state_load_failed = False
-        self.state_discard_result = False
+        self.state_discard_read = False
+        self.state_dirty = False
         self.state_waiting = False
 
         self.timeout_id = None
@@ -51,14 +72,15 @@ e.scrollTop = (e.scrollHeight - e.clientHeight) * scale;
         return self.scroll_scale
 
     def set_scroll_scale(self, scale):
-        self.pending_scroll_scale = scale
+        self.state_dirty = scale != self.scroll_scale
+        self.scroll_scale = scale
         self.state_loop()
 
     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_discard_read = event == WebKit2.LoadEvent.STARTED and self.state_waiting
+        self.state_dirty = True
         self.state_loop()
 
     def on_load_failed(self, _web_view, _event):
@@ -73,23 +95,16 @@ e.scrollTop = (e.scrollHeight - e.clientHeight) * scale;
         self.state_loaded = False
         self.state_loop()
 
-    def read_scroll_scale(self):
+    def sync_scroll_scale(self, scroll_scale, write):
         self.state_waiting = True
         self.run_javascript(
-            self.GET_SCROLL_SCALE_JS, None, self.finish_read_scroll_scale)
+            self.SYNC_SCROLL_SCALE_JS.format(scroll_scale, "true" if write else "false"),
+            None, self.finish_sync_scroll_scale)
 
-    def write_scroll_scale(self, scroll_scale):
-        self.run_javascript(
-            self.SET_SCROLL_SCALE_JS.format(scroll_scale), None, None)
-
-    def finish_read_scroll_scale(self, _web_view, result):
+    def finish_sync_scroll_scale(self, _web_view, result):
         self.state_waiting = False
-        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()
+        result = self.run_javascript_finish(result)
+        self.state_loop(result.get_js_value().to_double())
 
     def state_loop(self, scroll_scale=None, delay=16):  # 16ms ~ 60hz
         # Remove any pending callbacks
@@ -98,18 +113,17 @@ e.scrollTop = (e.scrollHeight - e.clientHeight) * scale;
             self.timeout_id = None
 
         # Set scroll scale if specified, and the state is not dirty
-        if scroll_scale not in (None, -1, self.scroll_scale):
+        if not self.state_discard_read and scroll_scale not in (None, -1, self.scroll_scale):
             self.scroll_scale = scroll_scale
             self.emit("scroll-scale-changed", self.scroll_scale)
+        else:
+            self.state_discard_read = False
 
         # Handle the current state
         if not self.state_loaded or self.state_load_failed or self.state_waiting:
             return
-        if self.pending_scroll_scale:
-            self.write_scroll_scale(self.pending_scroll_scale)
-            self.pending_scroll_scale = None
-            self.read_scroll_scale()
-        elif delay > 0:
-            self.timeout_id = GLib.timeout_add(delay, self.state_loop, None, 0)
+        elif self.state_dirty or delay == 0:
+            self.sync_scroll_scale(self.scroll_scale, self.state_dirty)
+            self.state_dirty = False
         else:
-            self.read_scroll_scale()
+            self.timeout_id = GLib.timeout_add(delay, self.state_loop, None, 0)