Avoid flashing during startup due to cover browser

During startup while the multiple resizes due to the various splitters
adjusting their sizes happens, paint the cover browser in the window
background color to avoid flashing.

src/calibre/gui2/cover_flow.py

@@ -13,12 +13,13 @@ import os
 import sys
 import time
 from qt.core import (
-    QAction, QApplication, QDialog, QFont, QImage, QItemSelectionModel,
+    QAction, QApplication, QDialog, QFont, QImage, QItemSelectionModel, QKeySequence,
-    QKeySequence, QLabel, QSize, QSizePolicy, QStackedLayout, Qt, QTimer, pyqtSignal
+    QLabel, QPainter, QPalette, QSize, QSizePolicy, QStackedLayout, Qt, QTimer,
+    pyqtSignal,
 )
 
 from calibre.constants import islinux
-from calibre.ebooks.metadata import rating_to_stars, authors_to_string
+from calibre.ebooks.metadata import authors_to_string, rating_to_stars
 from calibre.gui2 import config, gprefs, rating_font
 from calibre_extensions import pictureflow
 
@@ -186,10 +187,32 @@ class CoverFlow(pictureflow.PictureFlow):
 
     dc_signal = pyqtSignal()
     context_menu_requested = pyqtSignal()
+    _ignore_paint_events = False
+
+    @property
+    def ignore_paint_events(self):
+        return self._ignore_paint_events
+
+    @ignore_paint_events.setter
+    def ignore_paint_events(self, val):
+        if val != self._ignore_paint_events:
+            self._ignore_paint_events = val
+            if not val:
+                self.update()
+
+    def paintEvent(self, ev):
+        if self.ignore_paint_events and time.monotonic() - self.created_at < 1:
+            # Paint blank during startup to avoid flashing
+            p = QPainter(self)
+            p.fillRect(self.rect(), self.palette().color(QPalette.ColorRole.Window))
+            p.end()
+        else:
+            super().paintEvent(ev)
 
     def __init__(self, parent=None):
         pictureflow.PictureFlow.__init__(self, parent,
                             config['cover_flow_queue_length']+1)
+        self.created_at = time.monotonic()
         self.setMinimumSize(QSize(300, 150))
         self.setFocusPolicy(Qt.FocusPolicy.WheelFocus)
         self.setSizePolicy(QSizePolicy(QSizePolicy.Policy.Expanding,

src/calibre/gui2/widgets.py

@@ -1246,6 +1246,11 @@ class Splitter(QSplitter):
             # when the event loop ticks
             self.reapply_sizes.emit(sizes)
 
+    def ignore_child_paints(self, ignore=True):
+        for widget in self:
+            if hasattr(widget, 'ignore_paint_events'):
+                widget.ignore_paint_events = ignore
+
     def do_resize(self, *args):
         orig = self.desired_side_size
         super().resizeEvent(self._resize_ev)
@@ -1254,10 +1259,14 @@ class Splitter(QSplitter):
             while abs(self.side_index_size - orig) > 10 and c < 5:
                 self.apply_state(self.get_state(), save_desired=False)
                 c += 1
+        self.ignore_child_paints(False)
+
+    def __iter__(self):
+        for i in range(self.count()):
+            yield self.widget(i)
 
     def resizeEvent(self, ev):
-        if self.resize_timer.isActive():
+        self.ignore_child_paints()
-            self.resize_timer.stop()
         self._resize_ev = ev
         self.resize_timer.start()