private boolean openSecondProgram(DomainFile df) {
		if (!Program.class.isAssignableFrom(df.getDomainObjectClass())) {
			Msg.error(this, "Failed to launch Diff for non-Program file: " + df.getName());
			return false;
		}

		GetDomainObjectTask task =
			new GetDomainObjectTask(this, df, DomainFile.DEFAULT_VERSION, true);
		new TaskLauncher(task, tool.getToolFrame(), 500);
		// block until the task completes

		Program newProgram = (Program) task.getDomainObject();
		if (newProgram != null) {
			return openSecondProgram(newProgram, null);
		}
		return false;
	}
Reload the marked differences in the diff panel.
Ghidra/Features/ProgramDiff/src/main/java/ghidra/app/plugin/core/diff/ProgramDiffPlugin.java