Remove the opacity filters from !BackgroundIframe.  I think I put them in a long time ago to support semi-transparent dropdowns, but they aren't being used now, and on IE they are dangerous in terms of memory usage.  Fixes #16912 !strict.
Make a11yclick indicate the real event type so that _MenuBase can differentiate between keyboard and mouse clicks, the former of which should focus the popup menu.   Fixes #16904 !strict, although I could add more code to make ESC and down arrow work even when the dropdown menu was opened by the mouse.