Slow an initial load spinner while book is being rendered

src/calibre/gui2/viewer/ui.py

@@ -74,6 +74,7 @@ class EbookViewer(MainWindow):
 
     def load_ebook(self, pathtoebook, open_at=None):
         # TODO: Implement open_at
+        self.web_view.show_preparing_message()
         if self.save_annotations_debounce_timer.isActive():
             self.save_annotations()
         self.current_book_data = {}

src/calibre/gui2/viewer/web_view.py

@@ -169,6 +169,7 @@ class ViewerBridge(Bridge):
     set_session_data = from_js(object, object)
 
     create_view = to_js()
+    show_preparing_message = to_js()
     start_book_load = to_js()
 
 
@@ -315,10 +316,17 @@ class WebView(RestartingWebEngineView):
 
     def start_book_load(self, initial_cfi=None):
         key = (set_book_path.path,)
+        self.execute_when_ready('start_book_load', key, initial_cfi)
+
+    def execute_when_ready(self, action, *args):
         if self.bridge.ready:
-            self.bridge.start_book_load(key, initial_cfi)
+            getattr(self.bridge, action)(*args)
         else:
-            self.pending_bridge_ready_actions['start_book_load'] = key, initial_cfi
+            self.pending_bridge_ready_actions[action] = args
+
+    def show_preparing_message(self):
+        msg = _('Preparing book for reading, please wait…')
+        self.execute_when_ready('show_preparing_message', msg)
 
     def set_session_data(self, key, val):
         if key == '*' and val is None:

src/pyj/viewer-main.pyj

@@ -180,6 +180,12 @@ def create_view(prefs):
         create_session_data(prefs)
         view = View(document.getElementById('view'))
 
+
+@from_python
+def show_preparing_message(msg):
+    view.show_loading_message(msg)
+
+
 @from_python
 def start_book_load(key, initial_cfi):
     xhr = ajax('manifest', manifest_received.bind(None, key, initial_cfi), ok_code=0)