New window and new tab stopped working in WebKit r187880 on OS X 10.5.8. When WebKit immediately opens, they do work, but after some other activity they stop working. The "File menu" blinks briefly when using the 2 Commands shortcuts (Command-T and Command-N), but nothing happens. Relaunching WebKit application makes new window and new tab work again, for some time, then they stop working again. TenFourFox 45.8.0 and Safari 5.0.6 do not have this problem.
WebKit trunk does not support anything older than OS X 10.11 at this time.