GtkEntry and subclasses have two X windows. An inner one with the area of the text, and an outer one which is also filled with the base color. To draw rounded rectangles in themes it is commen to try and get the background color from some source (eg. walking up the widget tree) and filling the background of the outer window first. This works pretty good for normal GTK+ applications, but creates problems if the entry is used on a canvas (or eg. faked in mozilla).

To help with this it would be nice if the second window of the entry would be removed, or made input only. I do not know how feasable this may be (API and implementation wise).

Another solution that I thought about for mozilla is that mozilla attaches some data to the widget (eg. "gtk-engine-hint-transparent-bg") and the engine does not fill the background then. This may also be a feasable temporary workaround if that is desired. (The engines code could just drop the workaround code GTK+ is changed.)