fix(marks): undo reverts a mark set after the change #41330

Problem:
A named mark updated after a change is moved back (treated as the
original mark) by undo:

    :1mark d
    :$
    dw
    :2mark d   " 'd is on line 2
    :undo      " 'd is back on line 1

The undo header snapshots `b_namedm` when the change is recorded, and
`u_undoredo()` restores that snapshot indiscriminately.

Solution:
Update the pending header's snapshot when a mark is set explicitly.
Marks that the change itself moved go through mark_adjust(), not
setmark_pos(), so those are still reverted.

Similar to 2546741d1b (for extmarks): an explicit set inside an undo
block is confused with an edit-driven adjustment. But the extmarks case
is dealing with mid-edit moves, whereas named/regular marks only need
the stale snapshot dropped.

runtime/doc/news.txt

@@ -514,6 +514,9 @@ These existing features changed their behavior.
   `opts.plain=true` and now does not expand leading tildes ("~") in addition
   to environment variables ("expand_env" is still accepted, for backwards
   compatibility).
+• |undo| no longer restores the old (wrong) position of a mark that you moved
+  (|m|, |:mark|) later. The mark shifts with the text it was moved to, the
+  same as a mark the change never touched.
 
 ==============================================================================
 REMOVED FEATURES                                                 *news-removed*

runtime/doc/vim_diff.txt

@@ -682,6 +682,12 @@ Mappings:
 - "#" followed by a digit doesn't stand for a function key at the start of the
   lhs of a mapping.
 
+Marks:
+- |undo| does not restore a mark that you moved (|m|, |:mark|) after the change
+  being undone; the mark shifts with the text it was moved to, the same as a
+  mark the change never touched. Vim restores its old (wrong) position. Marks
+  moved by the change itself are still restored.
+
 Motion:
 - The |jumplist| avoids useless/phantom jumps.
 

src/nvim/mark.c

@@ -50,6 +50,7 @@
 #include "nvim/tag.h"
 #include "nvim/textobject.h"
 #include "nvim/types_defs.h"
+#include "nvim/undo.h"
 #include "nvim/vim_defs.h"
 
 // This file contains routines to maintain and manipulate marks.
@@ -184,6 +185,8 @@ int setmark_pos(int c, pos_T *pos, int fnum, fmarkv_T *view_pt)
   if (ASCII_ISLOWER(c)) {
     i = c - 'a';
     RESET_FMARK(buf->b_namedm + i, *pos, fnum, view);
+    // Moving a mark is not part of the pending change, so undo must not revert it. #5754
+    u_update_named_mark(buf, i);
     do_markset_autocmd((char)c, pos, buf);
     return OK;
   }

src/nvim/undo.c

@@ -3235,6 +3235,19 @@ void f_undotree(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
   tv_dict_add_list(dict, S_LEN("entries"), u_eval_tree(buf, buf->b_u_oldhead));
 }
 
+/// Drops named mark `idx` from the pending undo snapshot (if the user moved that mark after the
+/// change was recorded, undo must not put it back).
+///
+/// The mark is then left to mark_adjust(), so undo shifts it with its text, like other marks not
+/// touched by the change.
+void u_update_named_mark(buf_T *buf, int idx)
+{
+  u_header_T *uhp = buf->b_u_curhead != NULL ? buf->b_u_curhead : buf->b_u_newhead;
+  if (uhp != NULL) {
+    uhp->uh_namedm[idx].mark.lnum = 0;
+  }
+}
+
 // Given the buffer, Return the undo header. If none is set, set one first.
 // NULL will be returned if e.g undolevels = -1 (undo disabled)
 u_header_T *u_force_get_undo_header(buf_T *buf)

test/functional/editor/mark_spec.lua

@@ -41,6 +41,35 @@ describe('named marks', function()
     eq({ 4, 0 }, api.nvim_buf_get_mark(0, 'c'))
   end)
 
+  it('moved after a change follow their text through undo #5754', function()
+    command('edit ' .. file1)
+    command('1mark d')
+    command('$')
+    feed('dw')
+    command('2mark d')
+    eq({ 2, 0 }, api.nvim_buf_get_mark(0, 'd'))
+    command('undo')
+    eq({ 2, 0 }, api.nvim_buf_get_mark(0, 'd'))
+    command('redo')
+    eq({ 2, 0 }, api.nvim_buf_get_mark(0, 'd'))
+
+    -- A mark on a line the change deletes is cleared by the change; undo restores it.
+    api.nvim_buf_set_mark(0, 'e', 2, 2, {})
+    command('2delete')
+    eq({ 0, 2 }, api.nvim_buf_get_mark(0, 'e'))
+    command('undo')
+    eq({ 2, 2 }, api.nvim_buf_get_mark(0, 'e'))
+    eq('1test2', fn.getline(2))
+
+    -- A mark moved after a line-count change shifts with the text it was moved to.
+    command('2delete')
+    eq('1test3', fn.getline(2))
+    command('2mark f')
+    command('undo')
+    eq({ 3, 0 }, api.nvim_buf_get_mark(0, 'f'))
+    eq('1test3', fn.getline(3))
+  end)
+
   it('errors when set out of range with :mark', function()
     command('edit ' .. file1)
     local err = pcall_err(n.exec_capture, '1000mark x')