Semantic Highlighting Job should be canceled by the plugin that scheduled them during shutdown
The following warning is queued in the Error Log view if Eclipse is shutted down while the Semantic Highlighting Job is running:
Job found still running after platform shutdown. Jobs should be canceled by the plugin that scheduled them during shutdown: scala.tools.eclipse.semantichighlighting.Presenter$SemanticHighlightingJob
Stray Semantic Highlighting jobs (that appear to be running) prevent new jobs to be scheduled, leading to editors that have no semantic highlighting. This bad state can be triggered by changing the file while semantic highlighting is running.