The menu icon element is now an <html:img> that renders the 'image'
attribute or 'content: var(--menuitem-icon)', and 'list-style-image' is
ignored, so iconic menuitems showed broken-image placeholders on
Windows and Linux. list-style-image is kept for native macOS menus,
which still read it.
- on all platforms, Space will open the menulist popup
- on Windows, ArrowUp/ArrowDown will change selection without triggering command event.
This is needed to avoid alerts that are meant to fire after a confirmed
selection when the user is just trying to navigate the menu (e.g. item type menu)
- on Windows, fix odd appearance of checked menulist options in advanced search
condition dropdowns when "More" option is selected
- on Linux, fix menulist focus ring not appearing in Advanced Search
or Preferences (https://forums.zotero.org/discussion/129498/advanced-search-drop-down-menus-are-not-highlighted-during-keyboard-navigation)
Fixes: #5772
Firefox restyled Linux menupopups at some point to no longer conform to
the DE defined styles for GTK widgets. This removes that custom firefox
styling and uses DE defined values instead. This is especially apparent
on Linux Unity where menupopups are dark in both light and dark themes
by default for apps.
