Fix completion performance with shrink=True.

Before, the completion was shrinked every time any item was removed/added to
the completion (rowsRemoved/rowsInserted signals), which was >3000 times when
completing history.

Also, the signals got connected multiple times if setting the same model, which
made the situation worse.

Fixes #734.

CHANGELOG.asciidoc

@@ -81,6 +81,7 @@ Fixed
 - Fixed handling of keybindings containing Ctrl/Meta on OS X.
 - Fixed crash when downloading an URL without filename (e.g. magnet links) via "Save as...".
 - Fixed exception when starting qutebrowser with `:set` as argument.
+- Fixed horrible completion performance when the `shrink` option was set.
 
 https://github.com/The-Compiler/qutebrowser/releases/tag/v0.2.1[v0.2.1]
 -----------------------------------------------------------------------

qutebrowser/completion/completer.py

@@ -272,7 +272,7 @@ class Completer(QObject):
             pattern = parts[self._cursor_part].strip()
         except IndexError:
             pattern = ''
-        self._model().set_pattern(pattern)
+        completion.set_pattern(pattern)
 
         log.completion.debug(
             "New completion for {}: {}, with pattern '{}'".format(

qutebrowser/completion/completionwidget.py

@@ -201,8 +201,17 @@ class CompletionView(QTreeView):
         for i in range(model.rowCount()):
             self.expand(model.index(i, 0))
         self._resize_columns()
-        model.rowsRemoved.connect(self.maybe_resize_completion)
-        model.rowsInserted.connect(self.maybe_resize_completion)
+        self.maybe_resize_completion()
+
+    def set_pattern(self, pattern):
+        """Set the completion pattern for the current model.
+
+        Called from on_update_completion().
+
+        Args:
+            pattern: The filter pattern to set (what the user entered).
+        """
+        self.model().set_pattern(pattern)
         self.maybe_resize_completion()
 
     @pyqtSlot()