Did some investigation during the CTK hackfest and used the test code below to narrow down which widget is obscuring the thumbnails. It is a QScrollArea.

Note also that this issue goes away when resizing the widget.

This sounds familiar to Julien and may be related to a somewhat recent change he made to the way resize events are handles. Will re-assign this bug to him.

theWidget = None
for w in findChildren(DICOM.detailsPopup.window,name='*'):
try:
if w.visible:
w.setStyleSheet("%s { background-color: blue; }" % w.className());
#w.visible = False
print(w)
#w.visible = True
#qt.QTimer.singleShot(500,lambda : w)
#qt.QProcess.execute('sleep .5')
ret = qt.QMessageBox.information(mainWindow(), w.name, w.className() + " " + w.name, 0x400400)
if ret != 1024:
theWidget = w
break
w.setStyleSheet("%s { }" % w.className());
except AttributeError:
print('no widget for', w)

See https://github.com/commontk/CTK/issues/272