Committed fix at revision 36998.
The cause of the problem in IE is that unlike other browsers a blur event was being triggered when typing the first character after the popup becomes visible. What made things more difficult is that the onblur event seemed to be triggered before the onkeypress event.
Different approaches were tried to fix this issue, including executing the onblur event with a delay, in order to allow the onkeypress event set a flag and prevent the execution of the onblur event. Also, instead of timeouts, an approach comparing timestamps was tried. The event objects were also examined t try to find out differences between a real onblur event and the fake one, but it wasn't possible to determine this. The event objects for the onkeypress and onblur events were also different objects, with different timestamps, so it wasn't possible to determine when the onblur event was artificially triggered.
The solution consisted in adding a flag in the onkeypress event and allowing a delay of 500 ms to check in the onblur event whether it should be executed or not. This is only done on IE browsers when there's a modal frame running.
A fix for this issue was committed under ICE-8706.
It wasn't reproducible on the 1.8 trunk.