feat(clipboard)!: use OSC 52 as fallback clipboard provider (#31730)

We currently enable the OSC 52 clipboard provider by setting g:clipboard
when a list of conditions are met, one of which is that $SSH_TTY must be
set. We include this condition because often OSC 52 is not the best
clipboard provider, so if there are "local" providers available Nvim
should prefer those over OSC 52.

However, if no other providers are available, Nvim should use OSC 52
even when $SSH_TTY is not set. When a user is in an SSH session then the
checks for the other clipboard providers will still (typically) fail, so
OSC 52 continues to be enabled by default in SSH sessions.

This is marked as a breaking change because there are some cases where
OSC 52 wasn't enabled before and is now (or vice versa).

runtime/plugin/osc52.lua

@@ -6,7 +6,15 @@ for _, ui in ipairs(vim.api.nvim_list_uis()) do
   end
 end
 
-if not tty or vim.g.clipboard ~= nil or vim.o.clipboard ~= '' or not os.getenv('SSH_TTY') then
+-- Do not query when any of the following is true:
+--   * TUI is not attached
+--   * OSC 52 support is explicitly disabled via g:termfeatures
+--   * Using a badly behaved terminal
+if
+  not tty
+  or (vim.g.termfeatures ~= nil and vim.g.termfeatures.osc52 == false)
+  or vim.env.TERM_PROGRAM == 'Apple_Terminal'
+then
   return
 end
 
@@ -17,28 +25,13 @@ require('vim.termcap').query('Ms', function(cap, found, seq)
 
   assert(cap == 'Ms')
 
-  -- Check 'clipboard' and g:clipboard again to avoid a race condition
-  if vim.o.clipboard ~= '' or vim.g.clipboard ~= nil then
-    return
-  end
-
   -- If the terminal reports a sequence other than OSC 52 for the Ms capability
   -- then ignore it. We only support OSC 52 (for now)
   if not seq or not seq:match('^\027%]52') then
     return
   end
 
-  local osc52 = require('vim.ui.clipboard.osc52')
-
-  vim.g.clipboard = {
-    name = 'OSC 52',
-    copy = {
-      ['+'] = osc52.copy('+'),
-      ['*'] = osc52.copy('*'),
-    },
-    paste = {
-      ['+'] = osc52.paste('+'),
-      ['*'] = osc52.paste('*'),
-    },
-  }
+  local termfeatures = vim.g.termfeatures or {}
+  termfeatures.osc52 = true
+  vim.g.termfeatures = termfeatures
 end)