Ignore click events with invalid positions

Since Qt 5.11.2, various crash logs like this popped up after clicking a
<select> element:

    Traceback (most recent call last):
      File ".../browser/mouse.py", line 239, in eventFilter
        return self._handlers[evtype](event)
      File ".../browser/mouse.py", line 121, in _handle_mouse_press
        self._mousepress_insertmode_cb)
      File ".../browser/webengine/webenginetab.py", line 624, in find_at_pos
        assert pos.x() >= 0
    AssertionError

This is probably caused by https://codereview.qt-project.org/#/c/193908/ in
some way...

doc/changelog.asciidoc

@@ -96,6 +96,7 @@ Fixed
 - Crash when doing initial run on Wayland without XWayland
 - Crash when trying to load an empty session file.
 - `:hint` with an invalid `--mode=` value now shows a proper error.
+- Rare crash on Qt 5.11.2 when clicking on `<select>` elements.
 
 Removed
 ~~~~~~~

qutebrowser/browser/mouse.py

@@ -116,9 +116,13 @@ class MouseEventFilter(QObject):
 
         self._ignore_wheel_event = True
 
+        pos = e.pos()
+        if pos.x() < 0 or pos.y() < 0:
+            log.mouse.warning("Ignoring invalid click at {}".format(pos))
+            return False
+
         if e.button() != Qt.NoButton:
-            self._tab.elements.find_at_pos(e.pos(),
-                                           self._mousepress_insertmode_cb)
+            self._tab.elements.find_at_pos(pos, self._mousepress_insertmode_cb)
 
         return False
 

qutebrowser/browser/webengine/webenginetab.py

@@ -631,8 +631,8 @@ class WebEngineElements(browsertab.AbstractElements):
         self._tab.run_js_async(js_code, js_cb)
 
     def find_at_pos(self, pos, callback):
-        assert pos.x() >= 0
-        assert pos.y() >= 0
+        assert pos.x() >= 0, pos
+        assert pos.y() >= 0, pos
         pos /= self._tab.zoom.factor()
         js_code = javascript.assemble('webelem', 'find_at_pos',
                                       pos.x(), pos.y())