Terry Harper writes
My complaint about most modern double deck buses is not so much lack of
air conditioning, but larger windows with very little ventilation.
Effectively they are greenhouses on wheels...

The problem never existed when they had half-drop windows at the front
of the top deck.

Even Routemasters don't have those. You'll have to go all the way back
to the RT bus (a pre-WWII design) to get half-drop windows at the front
of the top-deck.

--
Dave