Windows: E-book viewer: Fix switching away from viewer while in fullscreen and switching back causing some corruption until the page is scrolled. Fixes #1918591 [[Viewer] Using Alt + Tab to switch programs in Windows when using the viewer in full screen, the text is adjusted as it was not in full screen](https://bugs.launchpad.net/calibre/+bug/1918591)

src/calibre/gui2/viewer/ui.py

@@ -17,7 +17,7 @@ from qt.core import (
 from threading import Thread
 
 from calibre import prints
-from calibre.constants import ismacos
+from calibre.constants import ismacos, iswindows
 from calibre.customize.ui import available_input_formats
 from calibre.db.annotations import merge_annotations
 from calibre.gui2 import choose_files, error_dialog, sanitize_env_vars
@@ -289,6 +289,7 @@ class EbookViewer(MainWindow):
 
     def toggle_full_screen(self):
         self.set_full_screen(not self.isFullScreen())
+
     # }}}
 
     # Docks (ToC, Bookmarks, Lookup, etc.) {{{
@@ -743,11 +744,16 @@ class EbookViewer(MainWindow):
         t.start()
 
     def eventFilter(self, obj, ev):
-        if ev.type() == QEvent.Type.MouseMove:
+        et = ev.type()
+        if et == QEvent.Type.MouseMove:
             if self.cursor_hidden:
                 self.cursor_hidden = False
                 QApplication.instance().restoreOverrideCursor()
             self.hide_cursor_timer.start()
+        elif et == QEvent.Type.FocusIn:
+            if iswindows and obj and obj.objectName() == 'EbookViewerClassWindow' and self.isFullScreen():
+                # See https://bugs.launchpad.net/calibre/+bug/1918591
+                self.web_view.repair_after_fullscreen_switch()
         return False
 
     def hide_cursor(self):

src/calibre/gui2/viewer/web_view.py

@@ -291,6 +291,7 @@ class ViewerBridge(Bridge):
     generic_action = to_js()
     show_search_result = to_js()
     prepare_for_close = to_js()
+    repair_after_fullscreen_switch = to_js()
     viewer_font_size_changed = to_js()
     tts_event = to_js()
 
@@ -742,3 +743,6 @@ class WebView(RestartingWebEngineView):
     def show_book_folder(self):
         path = os.path.dirname(os.path.abspath(set_book_path.pathtoebook))
         safe_open_url(QUrl.fromLocalFile(path))
+
+    def repair_after_fullscreen_switch(self):
+        self.execute_when_ready('repair_after_fullscreen_switch')

src/pyj/read_book/view.pyj

@@ -1255,11 +1255,13 @@ class View:
         }
         return pos
 
-    def show_status_message(self, msg):
+    def show_status_message(self, msg, timeout):
         self.current_status_message = msg or ''
         self.update_header_footer()
         if self.current_status_message:
-            window.setTimeout(def(): self.show_status_message();, 10000)
+            if not timeout?:
+                timeout = 10000
+            window.setTimeout(def(): self.show_status_message();, timeout)
 
     def create_template_renderer(self):
         if not self.book:

src/pyj/viewer-main.pyj

@@ -283,6 +283,12 @@ def prepare_for_close():
         ui_operations.close_prep_finished(None)
 
 
+@from_python
+def repair_after_fullscreen_switch():
+    if view:
+        view.show_status_message('...', 200)
+
+
 @from_python
 def viewer_font_size_changed():
     if view: