Cover Grid: Dynamically adjust the size of the in memory cover thumbnail cache based on the window size to ensure flicker free rendering even on very large screens. Fixes #1367211 [cover grid continuously redraws covers](https://bugs.launchpad.net/calibre/+bug/1367211)

src/calibre/gui2/__init__.py

@@ -117,7 +117,7 @@ defs['cover_grid_width'] = 0
 defs['cover_grid_height'] = 0
 defs['cover_grid_spacing'] = 0
 defs['cover_grid_color'] = (80, 80, 80)
-defs['cover_grid_cache_size'] = 100
+defs['cover_grid_cache_size_multiple'] = 5
 defs['cover_grid_disk_cache_size'] = 2500
 defs['cover_grid_show_title'] = False
 defs['cover_grid_texture'] = None

src/calibre/gui2/library/alternate_views.py

@@ -635,6 +635,9 @@ class GridView(QListView):
         self.update_timer.setInterval(200)
         self.update_timer.timeout.connect(self.update_viewport)
         self.update_timer.setSingleShot(True)
+        self.resize_timer = t = QTimer(self)
+        t.setInterval(200), t.setSingleShot(True)
+        t.timeout.connect(self.update_memory_cover_cache_size)
 
     @property
     def first_visible_row(self):
@@ -719,14 +722,30 @@ class GridView(QListView):
             self.delegate.calculate_spacing()
             self.setSpacing(self.delegate.spacing)
         self.set_color()
-        self.delegate.cover_cache.set_limit(gprefs['cover_grid_cache_size'])
         if size_changed:
             self.thumbnail_cache.set_thumbnail_size(self.delegate.cover_size.width(), self.delegate.cover_size.height())
         cs = gprefs['cover_grid_disk_cache_size']
         if (cs*(1024**2)) != self.thumbnail_cache.max_size:
             self.thumbnail_cache.set_size(cs)
+        self.update_memory_cover_cache_size()
+
+    def resizeEvent(self, ev):
+        self.resize_timer.start()
+        return QListView.resizeEvent(self, ev)
+
+    def update_memory_cover_cache_size(self):
+        try:
+            sz = self.delegate.item_size
+        except AttributeError:
+            return
+        rows, cols = self.width() // sz.width(), self.height() // sz.height()
+        num = (rows + 1) * (cols + 1)
+        limit = max(100, num * max(2, gprefs['cover_grid_cache_size_multiple']))
+        if limit != self.delegate.cover_cache.limit:
+            self.delegate.cover_cache.set_limit(limit)
 
     def shown(self):
+        self.update_memory_cover_cache_size()
         if self.render_thread is None:
             self.thumbnail_cache.set_database(self.gui.current_db)
             self.render_thread = Thread(target=self.render_covers)

src/calibre/gui2/preferences/look_feel.py

@@ -146,7 +146,7 @@ class ConfigWidget(ConfigWidgetBase, Ui_Form):
         r('bd_overlay_cover_size', gprefs)
         r('cover_grid_width', gprefs)
         r('cover_grid_height', gprefs)
-        r('cover_grid_cache_size', gprefs)
+        r('cover_grid_cache_size_multiple', gprefs)
         r('cover_grid_disk_cache_size', gprefs)
         r('cover_grid_spacing', gprefs)
         r('cover_grid_show_title', gprefs)

src/calibre/gui2/preferences/look_feel.ui

@@ -582,10 +582,10 @@ A value of zero means calculate automatically.</string>
               <item row="1" column="0" colspan="2">
                <widget class="QLabel" name="label_15">
                 <property name="text">
-                 <string>Number of covers to cache in &amp;memory (keep this small):</string>
+                 <string>Number of screenfulls of covers to cache in &amp;memory (keep this small):</string>
                 </property>
                 <property name="buddy">
-                 <cstring>opt_cover_grid_cache_size</cstring>
+                 <cstring>opt_cover_grid_cache_size_multiple</cstring>
                 </property>
                </widget>
               </item>
@@ -604,12 +604,15 @@ A value of zero means calculate automatically.</string>
                </widget>
               </item>
               <item row="1" column="2">
-               <widget class="QSpinBox" name="opt_cover_grid_cache_size">
+               <widget class="QSpinBox" name="opt_cover_grid_cache_size_multiple">
                 <property name="toolTip">
-                 <string>The maximum number of covers to keep in memory. Increasing this will make rendering faster, at the cost of more memory usage.</string>
+                 <string>The maximum number of screenfulls of thumbnails to keep in memory. Increasing this will make rendering faster, at the cost of more memory usage. Note that regardless of this setting, a minimum of one hudred thumbnails are always kept in memory, to ensure flicker free rendering.</string>
+                </property>
+                <property name="minimum">
+                 <number>2</number>
                 </property>
                 <property name="maximum">
-                 <number>50000</number>
+                 <number>100</number>
                 </property>
                </widget>
               </item>