If you wanted to retain the dropdown functionality you could create your own frame to handle the event.
UIParent:UnregisterEvent("GLOBAL_MOUSE_DOWN")
local f = CreateFrame("Frame")
f:RegisterEvent("GLOBAL_MOUSE_DOWN")
f:SetScript("OnEvent", function(self, event, ...)
UIParent:GetScript("OnEvent")(UIParent, "GLOBAL_MOUSE_UP", ...)
end)
There should only be one of these for the entire UI, you don’t want to be processing the same event doing exactly the same thing for every addon or widget etc.
GLOBAL_MOUSE_DOWN does some other things that also happen on GLOBAL_MOUSE_UP. This code would have little effect unless someone is prone to moving the mouse off the on-screen field/button before releasing the mouse button.