Bug 273489 [Mac] We should not nest sheets
bundles/org.eclipse.ui.ide/src/org/eclipse/ui/internal/ide/dialogs/PathVariableDialog.java
public final static int EXISTING_VARIABLE = 2;
    /**
     * Constructs a dialog for editing a new/existing path variable.
     * 
     * @param parentShell the parent shell
     * @param type the dialog type: <code>NEW_VARIABLE</code> or
     * 	<code>EXISTING_VARIABLE</code>
     * @param variableType the type of variable that can be edited in 
     * 	this dialog. <code>IResource.FILE</code> or <code>IResource.FOLDER</code>
     * @param pathVariableManager a reference to the path variable manager
     * @param namesInUse a set of variable names currently in use 
     */
    public PathVariableDialog(Shell parentShell, int type, int variableType,
            IPathVariableManager pathVariableManager, Set namesInUse) {
        super(parentShell);
        this.type = type;
        this.newVariable = type == NEW_VARIABLE;
        this.variableName = ""; 
        this.variableValue = ""; 
        this.variableType = variableType;
        this.pathVariableManager = pathVariableManager;
        this.namesInUse = namesInUse;
        if (newVariable) {
			this.standardMessage = IDEWorkbenchMessages.PathVariableDialog_message_newVariable;
		} else {
			this.standardMessage = IDEWorkbenchMessages.PathVariableDialog_message_existingVariable;
		}
        setShellStyle(getShellStyle() | SWT.SHEET);
    }
    /**