I found I can only maximize/restore windows with a double click if I make an unusual effort to click hard.
I found how to change the double click time for window titles.
LXDE button/ Preferences/ Openbox Configuration Manager / Mouse
Change doubleclick time to 300 - 500 ms.
I haven't found how to change it for PCManFM. The config file to do that seems to be missing on my Lubuntu.