The Source Pane shows the source code of the method associated with the current stack frame. The expression currently being executed is highlighted, unless the top stack frame is selected, in which case the highlight marks text of the expression which is about to be executed. Although this may seem confusing at first glance, it seems natural when using the Debugger to step through code.
You may choose to modify the source of the currently executing method and then recompile it using Workspace/Accept. If you do this, the method will be set to restart from the first statement when execution is resumed.