Running an application in IE9 does not necessarily mean you are running IE9 standards mode. IE9 has many modes that can be defined in the page head tag (see the "Document Mode" section below). You can overwrite the page mode manually by selecting F12, where you can set both browser mode and document mode.
GWT IE permutations work best with each version of "standards" mode. Mixing modes, say browser mode = 7 and document mode = 9, is not recommended and the behavior is undefined. To keep it simple, try to keep browser modes and document modes the same. If you must use mixed mode, be aware that you may run into issues that are still not supported. The exception is if you are emulating an older browser when you still do not support the new version, for instance, you emulate IE7 (EmulateIE7) on IE9.
Due to the many 'modes', when filling issues, make sure to add both the browser and document mode; and the browser version. This will help us triage what is IE9 specific, what is related to older versions or if the issue is related to mixed 'mode' setting.
It is important to understand how compatibility mode works before you release a new version of your app. If you are using X-UA-compatible tag, test on older browsers as well.
In short, whenever possible, use standards mode by adding <!DOCTYPE html> as the first element in your html file; and add <meta http-equiv="X-UA-Compatible" content="IE=9" > to <head> to future proof your app. Avoid using 'edge' unless you are sure what you are doing.
More information can be found here: http://msdn.microsoft.com/en-us/library/cc288325%28v=vs.85%29.aspx
In some cases IE9 will run in Software Rendering mode and bypass any hardware acceleration. In these cases you may see degraded performance. To see if you are running hardware accelerated environment, select "Tools > Internet Options > Advanced Tab."
More information can be found here: http://support.microsoft.com/kb/2398082
By default, all intranet sites are set to run in compatibility mode. To change that, go to "Tools > Compatibility View Settings" and uncheck "Display intranet sites in Compatibility View". Internet sites, except those added to the "Websiteslist you've added to Compatibility View" are by default in non-compatibility view.
Fall back bindings have been implemented, thus the introduction of the new user.agent should not cause build breaks. For instance, IE9 uses IE8 as fall back. If your application has a specific binding for IE8 (say a custom widget) that is not implemented for IE9, the compiler will automatically use the IE8 implementation. A warning will be raised during compilation (see below), and it's suggested that you verify the implementation works as expected in IE9.
Could not find an exact match rule. Using 'closest' rule <replace-with class='com.google.gwt.widget.client.impl.MySuperDuperWidgetIE6'/> based on fall back values. You may need to implement a specific binding in case the fall back behavior does not replace the missing binding
This is telling you that the IE9 permutation of your application will use MySuperDuperWidgetIE6. Your module binds IE8 to this implementation, and since there is no explicit binding for IE9, it will fall back to whatever binding IE8 is using (in this case a baseline IE6 implementation). The action item here is to verify that this implementation works as expected in IE9 standards mode.