Fix specifying options for layouts in the startup session file not working

Fixes #2520

docs/changelog.rst

@@ -22,6 +22,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 - diff kitten: New option to control the background color for filler lines in
   the margin (:iss:`2518`)
 
+- Fix specifying options for layouts in the startup session file not working
+  (:iss:`2520`)
+
 
 0.17.2 [2020-03-29]
 --------------------

kitty/session.py

@@ -53,7 +53,7 @@ def set_next_title(self, title: str) -> None:
         self.tabs[-1].next_title = title.strip()
 
     def set_layout(self, val: str) -> None:
-        if val not in all_layouts:
+        if val.partition(':')[0] not in all_layouts:
             raise ValueError('{} is not a valid layout'.format(val))
         self.tabs[-1].layout = val