I haven't been able to successfully use F5 while debugging grails application in STS 2.9.2.RELEASE because whenever there is a function call the IDE goes into numerous grails/groovy internal code lines and makes almost impossible to use step-into feature. Is there way to disable this behavior similarly to what is done for IntelliJ in this question? I can of course set up break points on first lines of the functions I'm expecting to be called and use F6 or F8, but that's not always convenient and not as efficient.
I'm not sure what version of eclipse your STS version might be built on, but in Juno with groovy-eclipse plugin, there are tweakables to this end in Preferences->Groovy->Debugger. Lacking that (or in addtion), the debugger may have "step filters" available as an option to turn on (in eclipse, this is available as a toolbar button while debugging). This should let you filter out stack frames involving classes of which you don't want to see details. (groovy-eclipse has a button to add in all the common groovy packages to the step filters; perhaps the same is true in STS).