Make jumping to a cfi pseudo synchronous by spinning the event loop while waiting for the scroll to finish

src/calibre/ebooks/oeb/display/paged.coffee

@@ -530,7 +530,7 @@ class PagedDisplay
             # selection
             this.scroll_to_xpos(left+5)
 
-    jump_to_cfi: (cfi) ->
+    jump_to_cfi: (cfi, job_id=-1) ->
         # Jump to the position indicated by the specified conformal fragment
         # indicator (requires the cfi.coffee library). When in paged mode, the
         # scroll is performed so that the column containing the position
@@ -540,6 +540,8 @@ class PagedDisplay
                 this.scroll_to_xpos(x)
             else
                 window.scrollTo(0, y)
+            if window.py_bridge
+                window.py_bridge.jump_to_cfi_finished(job_id)
         )
 
     current_cfi: () ->

src/calibre/gui2/viewer/documentview.py

@@ -93,6 +93,7 @@ class Document(QWebPage):  # {{{
         # javascript, get/set the value as: py_bridge.value
         self.bridge_value = None
         self.first_load = True
+        self.jump_to_cfi_listeners = set()
 
         self.debug_javascript = debug_javascript
         self.anchor_positions = {}
@@ -336,6 +337,11 @@ class Document(QWebPage):  # {{{
     def debug(self, msg):
         prints(unicode(msg))
 
+    @pyqtSlot(int)
+    def jump_to_cfi_finished(self, job_id):
+        for l in self.jump_to_cfi_listeners:
+            l(job_id)
+
     def reference_mode(self, enable):
         self.javascript(('enter' if enable else 'leave')+'_reference_mode()')
 

src/calibre/gui2/viewer/main.py

@@ -722,23 +722,16 @@ class EbookViewer(MainWindow):
             if self.isFullScreen():
                 self.relayout_fullscreen_labels()
         self.view.document.after_resize()
-        # For some reason scroll_fraction returns incorrect results in paged
-        # mode for some time after a resize is finished. No way of knowing
-        # exactly how long, so we update it in a second, in the hopes that it
-        # will be enough *most* of the time.
-        QTimer.singleShot(1000, self.scroll_after_resize_done)
         if not wmc:
             pre_footnote_pos = self.pre_footnote_toggle_position()
             if pre_footnote_pos is not None:
                 self.view.document.page_number = pre_footnote_pos
             else:
                 self.view.document.page_position.restore()
-
-    def scroll_after_resize_done(self):
-        self.update_page_number()
-        if len(self.page_position_on_footnote_toggle) % 2 == 1:
-            self.page_position_on_footnote_toggle[-1] = self.page_position_on_footnote_toggle[-1]._replace(
-                after_resize_page_number=self.view.document.page_number)
+                self.update_page_number()
+                if len(self.page_position_on_footnote_toggle) % 2 == 1:
+                    self.page_position_on_footnote_toggle[-1] = self.page_position_on_footnote_toggle[-1]._replace(
+                        after_resize_page_number=self.view.document.page_number)
 
     def update_page_number(self):
         self.set_page_number(self.view.document.scroll_fraction)

src/calibre/gui2/viewer/position.py

@@ -7,12 +7,19 @@ __license__   = 'GPL v3'
 __copyright__ = '2012, Kovid Goyal <kovid@kovidgoyal.net>'
 __docformat__ = 'restructuredtext en'
 
-import json
+import json, time
+
+from PyQt5.Qt import QApplication, QEventLoop
+
+from calibre.constants import DEBUG
 
 class PagePosition(object):
 
     def __init__(self, document):
         self.document = document
+        document.jump_to_cfi_listeners.add(self)
+        self.cfi_job_id = 0
+        self.pending_scrolls = set()
 
     @property
     def viewport_cfi(self):
@@ -30,9 +37,22 @@ class PagePosition(object):
 
     def scroll_to_cfi(self, cfi):
         if cfi:
+            jid = self.cfi_job_id
+            self.cfi_job_id += 1
             cfi = json.dumps(cfi)
+            self.pending_scrolls.add(jid)
             self.document.mainFrame().evaluateJavaScript(
-                    'paged_display.jump_to_cfi(%s)'%cfi)
+                    'paged_display.jump_to_cfi(%s, %d)' % (cfi, jid))
+            # jump_to_cfi is async, so we wait for it to complete
+            st = time.time()
+            WAIT = 1  # seconds
+            while jid in self.pending_scrolls and time.time() - st < WAIT:
+                QApplication.processEvents(QEventLoop.ExcludeUserInputEvents | QEventLoop.ExcludeSocketNotifiers)
+                time.sleep(0.01)
+            if jid in self.pending_scrolls:
+                self.pending_scrolls.discard(jid)
+                if DEBUG:
+                    print ('jump_to_cfi() failed to complete after %s seconds' % WAIT)
 
     @property
     def current_pos(self):
@@ -47,6 +67,9 @@ class PagePosition(object):
     def __exit__(self, *args):
         self.restore()
 
+    def __call__(self, cfi_job_id):
+        self.pending_scrolls.discard(cfi_job_id)
+
     def save(self, overwrite=True):
         if not overwrite and self._cpos is not None:
             return
@@ -66,5 +89,3 @@ class PagePosition(object):
 
     def set_pos(self, pos):
         self._cpos = pos
-
-