Input box on linux overwrites X clipboard

This is pretty annoying behaviour: I select some text, then talk to someone via Spark, then go to paste the text I’'d selected and discover that whatever I just keyed into the Spark input box has overwritten my X selection. It only happens with sent messages, not typing-in-progress ones. Spark 2.5.1, JRE 1.5.0 (SUN), Red Hat Enterprise Linux 3.