From bug 1159772 we know that nsIWidget::SetTransparencyMode calls can cause sync painting. So they aren't safe during reflow or frame construction. Bug 1159772 fixed on case, but menupopup frames still do it. http://mxr.mozilla.org/mozilla-central/source/layout/xul/nsMenuPopupFrame.cpp?rev=7f2cb4c27f48#313
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