Expand history completion results if on last index.

When tabbing to the last index of history completion, call expandAll
which will call fetchMore to retrieve more query results, if available.

Calling fetchMore directly will not update the view, and for some
reason self.expand(idx.parent()) and
self.expand(self.model().index(idx.row(), 0)) did not work, so I'm using
expandAll.

Fixes #2841.

qutebrowser/completion/completionwidget.py

@@ -255,6 +255,10 @@ class CompletionView(QTreeView):
         selmodel.setCurrentIndex(
             idx, QItemSelectionModel.ClearAndSelect | QItemSelectionModel.Rows)
 
+        # if the last item is focused, try to fetch more
+        if idx.row() == self.model().rowCount(idx.parent()) - 1:
+            self.expandAll()
+
         count = self.model().count()
         if count == 0:
             self.hide()

tests/unit/completion/test_completionwidget.py

@@ -65,6 +65,9 @@ def completionview(qtbot, status_command_stub, config_stub, win_registry,
     }
     # mock the Completer that the widget creates in its constructor
     mocker.patch('qutebrowser.completion.completer.Completer', autospec=True)
+    mocker.patch(
+        'qutebrowser.completion.completiondelegate.CompletionItemDelegate',
+        new=lambda *_: None)
     view = completionwidget.CompletionView(win_id=0)
     qtbot.addWidget(view)
     return view
@@ -185,6 +188,37 @@ def test_completion_item_focus_no_model(which, completionview, qtbot):
         completionview.completion_item_focus(which)
 
 
+def test_completion_item_focus_fetch(completionview, qtbot):
+    """Test that on_next_prev_item moves the selection properly.
+
+    Args:
+        which: the direction in which to move the selection.
+        tree: Each list represents a completion category, with each string
+              being an item under that category.
+        expected: expected argument from on_selection_changed for each
+                  successive movement. None implies no signal should be
+                  emitted.
+    """
+    model = completionmodel.CompletionModel()
+    cat = mock.Mock(spec=['layoutChanged', 'layoutAboutToBeChanged',
+        'canFetchMore', 'fetchMore', 'rowCount', 'index', 'data'])
+    cat.canFetchMore = lambda *_: True
+    cat.rowCount = lambda *_: 2
+    cat.fetchMore = mock.Mock()
+    model.add_category(cat)
+    completionview.set_model(model)
+    # clear the fetchMore call that happens on set_model
+    cat.reset_mock()
+
+    # not at end, fetchMore shouldn't be called
+    completionview.completion_item_focus('next')
+    assert not cat.fetchMore.called
+
+    # at end, fetchMore should be called
+    completionview.completion_item_focus('next')
+    assert cat.fetchMore.called
+
+
 @pytest.mark.parametrize('show', ['always', 'auto', 'never'])
 @pytest.mark.parametrize('rows', [[], ['Aa'], ['Aa', 'Bb']])
 @pytest.mark.parametrize('quick_complete', [True, False])