If you build your Qt app on Mac OS X the first time with the Cocoa version of Qt and you receive weird UI lockups / freezes while your CPU and memory is eaten step by step, ensure you replaced any existing
QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.4 QMAKE_MAC_SDK = /Developer/SDKs/MacOSX10.4u.sdk
with
QMAKE_MACOSX_DEPLOYMENT_TARGET = 10.5 QMAKE_MAC_SDK = /Developer/SDKs/MacOSX10.5.sdk
in your project file, because on Mac OS X 10.4 Qt Cocoa is officially unsupported. So if you need OSX 10.4 compatibility, stick with the Carbon version of Qt (4.7 will be the last version which comes with that), otherwise raise the bar to 10.5 or later and use the Cocoa build.