Don't instantiate completion models nedlessly.

For real this time. A mistake on the last commit like this meant models
were still spuriously instantiated.
Now that the completion model is reused, the layoutChanged signal needs
to be forwarded through, otherwise the view will not update.

qutebrowser/completion/completer.py

@@ -224,20 +224,19 @@ class Completer(QObject):
         if func is None:
             log.completion.debug('Clearing completion')
             completion.set_model(None)
-        elif func != self._last_completion_func:
+            self._last_completion_func = None
+            return
+
+        if func != self._last_completion_func:
             self._last_completion_func = func
             args = (x for x in before_cursor[1:] if not x.startswith('-'))
             with debug.log_time(log.completion,
-                    'Instantiate {} completion'.format(func.__name__)):
+                    'Starting {} completion'.format(func.__name__)):
                 model = func(*args)
             with debug.log_time(log.completion, 'Set completion model'):
                 completion.set_model(model)
-            completion.set_pattern(pattern)
-        else:
-            log.completion.debug('Setting pattern {}'.format(pattern))
-            completion.set_pattern(pattern)
 
-        self._last_completion_func = None
+        completion.set_pattern(pattern)
 
     def _change_completed_part(self, newtext, before, after, immediate=False):
         """Change the part we're currently completing in the commandline.

qutebrowser/completion/completionwidget.py

@@ -283,8 +283,6 @@ class CompletionView(QTreeView):
         self._column_widths = model.column_widths
         self._active = True
 
-        self.set_pattern('')
-
         if (config.get('completion', 'show') == 'always' and
                 model.count() > 0):
             self.show()

qutebrowser/completion/models/completionmodel.py

@@ -63,6 +63,7 @@ class CompletionModel(QAbstractItemModel):
     def add_category(self, cat):
         """Add a completion category to the model."""
         self._categories.append(cat)
+        cat.layoutChanged.connect(self.layoutChanged)
 
     def data(self, index, role=Qt.DisplayRole):
         """Return the item data for index.

tests/unit/completion/test_completionmodel.py

@@ -56,7 +56,7 @@ def test_first_last_item(counts):
 def test_count(counts):
     model = completionmodel.CompletionModel()
     for c in counts:
-        cat = mock.Mock(spec=['rowCount'])
+        cat = mock.Mock(spec=['rowCount', 'layoutChanged'])
         cat.rowCount = mock.Mock(return_value=c)
         model.add_category(cat)
     assert model.count() == sum(counts)
@@ -66,7 +66,7 @@ def test_count(counts):
 def test_set_pattern(pat):
     """Validate the filtering and sorting results of set_pattern."""
     model = completionmodel.CompletionModel()
-    cats = [mock.Mock(spec=['set_pattern'])] * 3
+    cats = [mock.Mock(spec=['set_pattern', 'layoutChanged'])] * 3
     for c in cats:
         c.set_pattern = mock.Mock()
         model.add_category(c)