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
}
	/**
	 * Computes and returns the source reference that includes the caret and
	 * serves as provider for the outline page selection and the editor range
	 * indication.
	 * 
	 * @return the computed source reference
	 * @since 3.0
	 */
	protected ISourceReference computeHighlightRangeSourceReference() {
		ISourceViewer sourceViewer= getSourceViewer();
		if (sourceViewer == null)
			return null;
			
		StyledText styledText= sourceViewer.getTextWidget();
		if (styledText == null)
			return null;
		
		int caret= 0;
		if (sourceViewer instanceof ITextViewerExtension5) {
			ITextViewerExtension5 extension= (ITextViewerExtension5)sourceViewer;
			caret= extension.widgetOffset2ModelOffset(styledText.getCaretOffset());
		} else {
			int offset= sourceViewer.getVisibleRegion().getOffset();
			caret= offset + styledText.getCaretOffset();
		}
		IJavaElement element= getElementAt(caret, false);
		
		if ( !(element instanceof ISourceReference))
			return null;