I use GTK for windows because windows is my favorite development platform, but in production code GTK is only used on Unix platforms (Linux / FreeBSD). Its just bad for production code on Windows7. The umlaute key errors have already been mentioned so as to add problems with pop-ups and focus events.
Also, even though it's called multipattform on the GTK webpage, the team doesn't care too much about Windows and MacOSX. This hostile attitude makes it much worse and more dangerous to use code. A good sign is that they just didn't come up with an easy way to compile the VS2010 environment. I do not understand that it is so difficult to write simple make files.
There is no chance on Windows other than using WPF or MFC. And on MacOSX you have to use Cocoa. Windows7 and Cocoa both have outdated large parts of the abstract engine, which means (and this is a clear strategic goal) developers should use the platform GUI tools in the long run.
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