A drop-down selector, like a popup menu, is traditionally a window-modal (or task-modal) UI element that can be dismissed with a click elsewhere (which is ignored), or pressing Escape (which is ignored). (Try this in e.g. the web browser you're reading it in).
PulseView channel configuration popups are also dismissed if you dismiss the drop-down, which is annoying because you can rapidly configure the channel using keyboard alone, switching between fields with Tab, but if you open the wrong drop-down selector and dismiss it with Escape, you have to hunt down the channel all over again, with mouse, or, worse, touchpad on a laptop.
Thanks for the bug report, will revisit this issue again after the change to QtQUick2/QML since I'll need to completely rework the entire UI for that anyway.