Give error when trying to detach a single tab

qutebrowser/browser/commands.py

@@ -424,6 +424,8 @@ class CommandDispatcher:
     @cmdutils.register(instance='command-dispatcher', scope='window')
     def tab_detach(self):
         """Detach the current tab to its own window."""
+        if self._count() < 2:
+            raise cmdexc.CommandError("Cannot detach one tab.")
         url = self._current_url()
         self._open(url, window=True)
         cur_widget = self._current_widget()

tests/end2end/features/tabs.feature

@@ -620,6 +620,12 @@ Feature: Tab management
               - history:
                 - url: http://localhost:*/data/numbers/2.txt
 
+    Scenario: Detach tab from window with only one tab
+        Given I have a fresh instance
+        When I open data/hello.txt
+        And I run :tab-detach
+        Then the error "Cannot detach one tab." should be shown
+
     # :undo
 
     Scenario: Undo without any closed tabs