Hi Eli and Stefan,

Attaching the latest iteration of the patch (renamed 
0003-Add-context-menu-share-to-mouse.el.patch) incorporating your 
feedback.

New in patch:

- Sharing/opening file externally is now generic and available via 
  context menu (needs context-menu-mode enabled) on multiple 
  platforms.
- Added file opening/sharing support for GNU/Linux via (xdg-open) 
  and any other platform which support it.
  - Region/text opening/sharing is now supported by xdg-open too.
- Sharing/opening routing behavior is now configurable and can 
  override the context menu implementation.
- Removes lambda usage from Objective-C/native code.
- Guards macOS addition via NS_IMPL_COCOA (avoid breaking 
  GNUstep).

While routing logic is now configurable, it'be great to extend the 
default behavior for other platforms. Android sounds like a great 
candidate as per Rudolf's suggested API 
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=76120#47. Any 
Android folks keen on extending the feature?

Also attaching screenshots as per current patch for both GNU/Linux 
and macOS.

Álvaro