Prevent sidebar toolbar from folding

If there is little space, Qt replaces some tools of the toolbar with an
expandable menu indicated by an arrow. But this arrow is invisible in
night mode.

qt/aqt/browser/sidebar/toolbar.py

@@ -30,6 +30,7 @@ class SidebarToolbar(QToolBar):
         qconnect(self._action_group.triggered, self._on_action_group_triggered)
         self._setup_tools()
         self.setIconSize(QSize(16, 16))
+        self.setSizePolicy(QSizePolicy.Fixed, QSizePolicy.Fixed)
         self.setStyle(QStyleFactory.create("fusion"))
 
     def _setup_tools(self) -> None: