Bug 81983 [preferences] Disabling and enabling folding on small file takes forever
3.1 M4 Using enable/disable folding on a small CU takes forever. This is caused by the recent change to apply the preference to all Java editors. In another (hidden) window (Debug) I had StyledText open which (understandably) took long to react on the preference change. Non-visible editor's should not react to this change until they become visible.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/JavaEditor.java
* @see org.eclipse.ui.texteditor.AbstractTextEditor#performRevert()
	 */
	protected void performRevert() {
		ProjectionViewer projectionViewer= (ProjectionViewer) getSourceViewer();
		projectionViewer.setRedraw(false);
		try {
			
			boolean projectionMode= projectionViewer.isProjectionMode();
			if (projectionMode) {
				projectionViewer.disableProjection();				
				if (fProjectionModelUpdater != null)
					fProjectionModelUpdater.uninstall();
			}
			
			super.performRevert();
			
			if (projectionMode) {
				if (fProjectionModelUpdater != null)
					fProjectionModelUpdater.install(this, projectionViewer);	
				projectionViewer.enableProjection();
			}
			
		} finally {
			projectionViewer.setRedraw(true);
		}
	}
	
	/*
	 * @see org.eclipse.ui.texteditor.AbstractTextEditor#rulerContextMenuAboutToShow(org.eclipse.jface.action.IMenuManager)
	 */