Toolbarbutton with autodisable="self" property affects Messagebox z-index.

I have just recently found an issue with z-index recalculation that happens after opening a Messagebox.

Scenario: Create a main page (test.zul) with one button that opens Modal window (testModal.zul). Modal window should contain toolbarbutton that opens Messagebox.
1) Open Main window (test.zul)
2) Click on the button to open Modal window (testModal.zul)
3) Click on a toolbarbutton to Open Messagebox

Expected result: Messagebox is active. Its shadow covers Modal and Main windows

Actual Result: Messagebox goes behind Modal window

What I could find is that z-index of Modal window was changed to 1802. In Chrome z-index of a Modal window remains 1800

I have tried to override z-index of Modal window directly through zindex attribute or css with z-index=1800, but it becomes automatically overridden right after I open Messagebox.

Tested in Chrome and IE 11. The issue is only reproducible in IE 11.
ZK 7.0.2.FL.20140410

<window height="100%" width="100%" position="center, center">
    <button label="Open Modal Window">
    <attribute name="onClick">
        ((Window) Executions.createComponents("testModal.zul",self.getParent(),null)).doModal();
    </attribute>
    </button>
</window>

<window vflex="min" width="400px" position="center, center" closable="true" title="Test Modal Win">
    <toolbarbutton label="Open Messagebox" autodisable="self">
    <attribute name="onClick">
        Messagebox.show("Test", "Test", Messagebox.OK, Messagebox.EXCLAMATION);
    </attribute>
    </toolbarbutton>
    <button label="Open Messagebox" autodisable="self">
    <attribute name="onClick">
        Messagebox.show("Test", "Test", Messagebox.OK, Messagebox.EXCLAMATION);
    </attribute>
    </button>
</window>