Caused by rBc24be7ec6e5.
Before rBc24be7ec6e5, wm_handlers_do always called handlers a second time with event value KM_PRESS in case of a double click. After it, this was only the case for non-LEFTMOUSE events.
Since ui_popup_handler (almost) always returned WM_UI_HANDLER_BREAK, the second handler iteration with KM_PRESS wouldn't run. This fix just makes sure we return WM_UI_HANDLER_CONTINUE for double click events instead (causing second iteration to run).