Example: recent change to JavaScriptCore/wtf/MessageQueue.h broke the Windows build because MessageQueue.h is not included anywhere in JavaScriptCore so the project JavaScriptCore was not build. this project has "post-build step" which make copy of header files to the build directory from where the subsequent WebCore build includes JSC headers. As a result, the stale header of MessageQueue.h was used and build broke. Changing a cpp file from wtf caused recompile of JavaScriptCore and updated the headers. I'm not sure what the right fix for this is.
The right fix is to move all header copying into a Makefile-based project (like JavaScriptCoreGenerated). Makefile-based projects run on every build regardless of whether any .cpp files changed.