Bug 53582 Rebuild Project are not run in the background?
The "Build All" and "Rebuild All" actions are run in the background, but the "Build Project" and "Rebuild Project" actions aren't. This is not consistent and I was surprised to get a modal dialog when performing a "Rebuild Project".
bundles/org.eclipse.ui.ide/extensions/org/eclipse/ui/actions/BuildAction.java
for (int k = 0; k < editors.length; k++) {
				IEditorPart editor = editors[k];
				IEditorInput input = editor.getEditorInput();
				if (input instanceof IFileEditorInput) {
					IFile inputFile = ((IFileEditorInput)input).getFile();
					if (projects.contains(inputFile.getProject())) {
						page.saveEditor(editor, false);
					}
				}
			}
		}
	}
}
/* (non-Javadoc)
 * Method declared on WorkspaceAction.
 */
boolean shouldPerformResourcePruning() {
	return true;
}
/**
 * The <code>BuildAction</code> implementation of this
 * <code>SelectionListenerAction</code> method ensures that this action is
 * enabled only if all of the selected resources have buildable projects.
 */
protected boolean updateSelection(IStructuredSelection s) {
	projectsToBuild = null;
	return super.updateSelection(s) && getProjectsToBuild().size() > 0;
}
}