QCleanlooksStyle prevents the checkmarks to be visible inside the menu combobox.
See qcleanlooksstyle.cpp:

        bool checkable = menuItem->checkType != QStyleOptionMenuItem::NotCheckable;
        bool checked = menuItem->checked;
        bool sunken = menuItem->state & State_Sunken;
        bool enabled = menuItem->state & State_Enabled;

        bool ignoreCheckMark = false;
        int checkcol = qMax(menuItem->maxIconWidth, 20);

#ifndef QT_NO_COMBOBOX
if (qobject_cast<const QComboBox*>(widget))
ignoreCheckMark = true; //ignore the checkmarks provided by the QComboMenuDelegate
#endif

        if (!ignoreCheckMark) {

Maybe it might work to:

• create a proxy style that checks the type of base style (qcleanlooks) and
• pass 0 (-> null) as the widget parameter for the drawControl(CE_MenuItem...) function

Another problem will then rise, how to "check" all the items, for now, qMRMLNodeComboBoxMenuDelegate does:

• menuOption.checked = this->mCombo->currentIndex() == index.row();
  Which is incompatible with what qMRMLCheckableNodeComboBox does.

Quick fix in r18765:
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&amp;revision=18765

Need more care though. Can be taken care of later.

Is this still an issue?

Haven't checked with more recent Qt versions, but I would say it is still an issue. Very low priority though.

@finetjul: Given the recent work associated with the Slicer style. Is this still an issue ?

Yes, but it is an extremely low priority issue as a work around has been setup (see 0003393).

Commenting line added by r18765, the checkbox do NOT appear. As mentioned by Julien, this should be fixed at some point if it become a show stopper.

Since a workaroud has been implemented, un-targeting this issue.

Hi the issue is present again in Qt5 and the workaround I think does not work anymore or needs some fixes.

I was also able to reproduce this. I'm changing its priority to Normal from Low as after switching to Qt5 after the release of 4.8.1 this will be an issue in many Slicer modules.

@johan.andruejol you have fixed this in https://github.com/Slicer/Slicer/pull/864, we can close, can't we?

I'm closing this one too. Not sure if @finetjul still keeps track of the Slicer Mantis, but if you have anything to add, please feel free to do so.