fix(multicursor): clipboard=unnamed, explicit "+yy #41804

Problem:
- With `clipboard=unnamed[plus]`, per-cursor yanks are joined into '"'
  on exit, but not the clipboard.
- Multicursor `"+yy` calls the clipboard provider per-cursor, behaving
  as "last wins".

Solution:
- Never cascade to/from the clipboard provider.
- Implicit clipboard (`clipboard=unnamed[plus]`) uses the cursor-local
  unnamed register, so the cascade needs no `start_batch_changes()`. On
  exit, the clipboard gets the same joined result as the `"` register.
- Operations that reference the explicit clipboard registers `"+` / `"*`
  while multicursor is active, use the cached (primary) clipboard value.

runtime/doc/repeat.txt

@@ -447,8 +447,13 @@ LIMITATIONS						*mcursor-limitations*
 - During Insert-mode completion, live mirroring pauses if a preview could
   shift or overlap the primary's completion (same-line edits before the
   primary, or line insertions/deletions before it).
-- Cursors on the same line can be lost or misplaced by an edit that shifts
-  columns (e.g. inserting text).
+- Explicit clipboard registers (`"*yy`, `"+yy`) are global, not per-cursor. On
+  multicursor exit, you can set the clipboard to the joined yank: >
+      :let @+=@"
+- Implicit 'clipboard' ("unnamed"/"unnamedplus") updates the clipboard on
+  multicursor exit.
+- Same-line cursors may be lost/misplaced by an edit that shifts columns (e.g.
+  inserting text).
 - Time-travel undo (|g-|, |g+|, |:earlier|) clears all cursors.
 - Reloading/unloading a buffer (|:edit!|, 'autoread') clears its cursors
   (extmarks limitation).

src/nvim/clipboard.c

@@ -7,6 +7,7 @@
 #include "nvim/clipboard.h"
 #include "nvim/eval.h"
 #include "nvim/eval/typval.h"
+#include "nvim/mcursor.h"
 #include "nvim/option_vars.h"
 #include "nvim/register.h"
 
@@ -26,8 +27,8 @@ static bool clipboard_didwarn = false;
 /// @param quiet Suppress error messages
 /// @param writing if we're setting the contents of the clipboard
 ///
-/// @returns the yankreg that should be written into, or `NULL`
-/// if the register isn't a clipboard or provider isn't available.
+/// @return The yankreg that should be written-to/read-from, or `NULL` if the register isn't
+/// a clipboard or provider isn't available.
 yankreg_T *adjust_clipboard_name(int *name, bool quiet, bool writing)
 {
 #define MSG_NO_CLIP "clipboard: No provider. " \
@@ -84,6 +85,7 @@ end:
 
 bool get_clipboard(int name, yankreg_T **target, bool quiet)
 {
+  const bool implicit = name == NUL;  // clipboard=unnamed[plus]
   // show message on error
   bool errmsg = true;
 
@@ -91,6 +93,13 @@ bool get_clipboard(int name, yankreg_T **target, bool quiet)
   if (reg == NULL) {
     return false;
   }
+  if (mc_replaying()) {  // Multicursor cascade: don't read the provider.
+    if (implicit) {
+      return false;  // clipboard=unnamed[plus] behaves as (cursor-local) unnamed register.
+    }
+    *target = reg;  // Explicit "+/"* gets the primary cursor's (cached) clipboard value.
+    return true;
+  }
   free_register(reg);
 
   list_T *const args = tv_list_alloc(1);
@@ -202,7 +211,8 @@ err:
 
 void set_clipboard(int name, yankreg_T *reg)
 {
-  if (!adjust_clipboard_name(&name, false, true)) {
+  if (mc_replaying()  // Multicursor cascade never writes the clipboard.
+      || !adjust_clipboard_name(&name, false, true)) {
     return;
   }
 

src/nvim/mcursor.c

@@ -462,8 +462,6 @@ static void mc_cascade(void)
       return;
     }
   }
-  // Optimization: one clipboard-provider sync for the whole cascade.
-  start_batch_changes();
   McSandbox sb;
   mc_sandbox_enter(&sb, edits);
 
@@ -489,7 +487,6 @@ static void mc_cascade(void)
 done:
   atoms_free(&g_atoms);
   mc_sandbox_leave(&sb);
-  end_batch_changes();
   mc_cleanup(true);
   if (handle_get_buffer(sb.bufnr) == curbuf && !curbuf->b_u_synced
       && curbuf->b_u_newhead != NULL) {
@@ -1033,6 +1030,10 @@ static void mc_reg_gather(void)
     if (gather[i] && kv_size(joined[i]) > 0) {
       write_reg_contents_ex(MC_REGS[i], joined[i].items, (ssize_t)kv_size(joined[i]), false,
                             kMTLineWise, 0);
+      if (MC_REGS[i] == '"') {
+        // If implicit clipboard is enabled (clipboard=unnamed[plus]): write the joined result.
+        set_clipboard(NUL, get_y_previous());
+      }
     }
     kv_destroy(joined[i]);
   }

src/nvim/register.c

@@ -71,6 +71,9 @@ yankreg_T *get_y_register(int reg)
   return &y_regs[reg];
 }
 
+/// What `p` pastes: the last-written register (x: -, yy: 0, "+yy: +, :let @"=…: 0).
+///
+/// Exception: with clipboard=unnamed/unnamedplus, `p` tries the provider first.
 yankreg_T *get_y_previous(void)
   FUNC_ATTR_PURE
 {

test/functional/editor/mcursor_spec.lua

@@ -2891,9 +2891,9 @@ describe('multicursor', function()
       eq({ '', '' }, get_lines())
     end)
 
-    it("perf: provider syncs once per cascade with 'clipboard'", function()
+    it('clipboard: implicit clipboard=unnamed[plus], explicit "+', function()
       n.exec_lua([[
-        _G.copies = 0
+        _G.copies, _G.pastes = 0, 0
         _G.content = {}
         vim.g.clipboard = {
           name = 'test',
@@ -2905,20 +2905,40 @@ describe('multicursor', function()
           },
           paste = {
             ['+'] = function()
+              _G.pastes = _G.pastes + 1
               return _G.content
             end,
           },
         }
-        vim.o.clipboard = 'unnamedplus'
       ]])
+      local function provider()
+        return n.exec_lua('return { _G.copies, _G.pastes, _G.content }')
+      end
+
+      -- Implicit clipboard (clipboard=unnamed[plus]) is ignored during cascade.
+      -- On multicursor exit, the unnamed (") join is written to the clipboard.
+      command('set clipboard=unnamedplus')
       cursors({ 'aa bb', 'cc dd', 'ee ff' })
-      local base = n.exec_lua('return _G.copies')
       feed('dw')
       eq({ 'bb', 'dd', 'ff' }, get_lines())
-      -- One provider sync for the primary's own delete, ONE for the whole
-      -- cascade (not one per cursor), and the primary's registers win.
-      eq(base + 2, n.exec_lua('return _G.copies'))
-      eq({ 'ee ' }, n.exec_lua('return _G.content'))
+      -- Clipboard provider was updated only by the primary's "dw". Not cascaded.
+      eq({ 1, 0, { 'ee ' } }, provider())
+      feed('p') -- Pastes unnamed reg (not clipboard) per-cursor.
+      eq({ 'baa b', 'dcc d', 'fee f' }, get_lines())
+      eq({ 1, 1, { 'ee ' } }, provider())
+      clear_cursors() -- Exit: the joined yank is written to the clipboard.
+      eq({ 2, 1, { 'aa ', 'cc ', 'ee ', '' } }, provider())
+
+      -- Explicit clipboard "+".
+      command('set clipboard=')
+      cursors({ 'aa', 'bb' }, 'Qj0')
+      feed('"+yy') -- Explicit "+: writes the primary's yank to the clipboard.
+      eq({ 3, 1, { 'bb', '' } }, provider())
+      feed('"+p') -- Explicit "+: pastes the primary's clipboard at every cursor.
+      eq({ 'aa', 'bb', 'bb', 'bb' }, get_lines())
+      eq({ 3, 2, { 'bb', '' } }, provider())
+      clear_cursors()
+      eq({ 3, 2, { 'bb', '' } }, provider())
     end)
   end)