This is a bug. It should be working and it does in many other situations. When I try it, I do see that an exception is being thrown:
org.eclipse.core.runtime.OperationCanceledException
at org.codehaus.groovy.eclipse.codeassist.processors.GroovyProposalTypeSearchRequestor.checkCancel(GroovyProposalTypeSearchRequestor.java:356)
at org.codehaus.groovy.eclipse.codeassist.processors.GroovyProposalTypeSearchRequestor.processAcceptedPackages(GroovyProposalTypeSearchRequestor.java:618)
at org.codehaus.groovy.eclipse.codeassist.processors.PackageCompletionProcessor.generateProposals(PackageCompletionProcessor.java:56)
at org.codehaus.groovy.eclipse.codeassist.requestor.GroovyCompletionProposalComputer.computeCompletionProposals(GroovyCompletionProposalComputer.java:162)
at org.grails.ide.eclipse.editor.gsp.adapter.CodeCompletionDelegate.codeComplete(CodeCompletionDelegate.java:71)
at org.codehaus.jdt.groovy.model.GroovyCompilationUnit.codeComplete(GroovyCompilationUnit.java:598)
at org.eclipse.jdt.internal.core.CompilationUnit.codeComplete(CompilationUnit.java:359)
at org.eclipse.jst.jsp.ui.internal.contentassist.JSPJavaCompletionProposalComputer.computeJavaCompletionProposals(JSPJavaCompletionProposalComputer.java:237)
at org.eclipse.jst.jsp.ui.internal.contentassist.JSPJavaCompletionProposalComputer.computeCompletionProposals(JSPJavaCompletionProposalComputer.java:114)
at org.eclipse.wst.sse.ui.internal.contentassist.CompletionProposalComputerDescriptor.computeCompletionProposals(CompletionProposalComputerDescriptor.java:284)
at org.eclipse.wst.sse.ui.internal.contentassist.CompletionProposalCategory.computeCompletionProposals(CompletionProposalCategory.java:290)
at org.eclipse.wst.sse.ui.contentassist.StructuredContentAssistProcessor.collectProposals(StructuredContentAssistProcessor.java:475)
at org.eclipse.wst.sse.ui.contentassist.StructuredContentAssistProcessor.computeCompletionProposals(StructuredContentAssistProcessor.java:254)
at org.eclipse.wst.sse.ui.internal.contentassist.CompoundContentAssistProcessor.computeCompletionProposals(CompoundContentAssistProcessor.java:127)
at org.eclipse.jface.text.contentassist.ContentAssistant.computeCompletionProposals(ContentAssistant.java:1839)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup.computeProposals(CompletionProposalPopup.java:566)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup.access$16(CompletionProposalPopup.java:563)
at org.eclipse.jface.text.contentassist.CompletionProposalPopup$2.run(CompletionProposalPopup.java:498)
...
I'll have to see what is going on here.
UPDATE:
Raised this issue: https://issuetracker.springsource.com/browse/STS-3337
Found the problem and pushed a fix. Will be available in next snapshot build.