Disallow caret mode with QtWebEngine

qutebrowser/keyinput/modeman.py

@@ -265,10 +265,17 @@ class ModeManager(QObject):
             m = usertypes.KeyMode[mode]
         except KeyError:
             raise cmdexc.CommandError("Mode {} does not exist!".format(mode))
+
+        backend = usertypes.arg2backend[objreg.get('args').backend]
         if m in [usertypes.KeyMode.hint, usertypes.KeyMode.command,
                  usertypes.KeyMode.yesno, usertypes.KeyMode.prompt]:
             raise cmdexc.CommandError(
                 "Mode {} can't be entered manually!".format(mode))
+        elif (m == usertypes.KeyMode.caret and
+              backend == usertypes.Backend.QtWebEngine):
+            raise cmdexc.CommandError("Caret mode is not supported with "
+                                      "QtWebEngine yet.")
+
         self.enter(m, 'command')
 
     @pyqtSlot(usertypes.KeyMode, str, bool)