fix(inccommand): don't save information of a buffer twice (#24501)

Problem:    'inccommand' doesn't restore 'undolevels' properly for a
            buffer shown in multiple windows.
Solution:   Don't save information of a buffer twice.

2 files changed, 21 insertions, 13 deletions

diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index 5f1f5d5adc..ece6257c4a 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -2333,6 +2333,8 @@ static void cmdpreview_restore_undo(const CpUndoInfo *cp_undoinfo, buf_T *buf)
 static void cmdpreview_prepare(CpInfo *cpinfo)
   FUNC_ATTR_NONNULL_ALL
 {
+  Set(ptr_t) saved_bufs = SET_INIT;
+
   kv_init(cpinfo->buf_info);
   kv_init(cpinfo->win_info);
 
@@ -2344,19 +2346,19 @@ static void cmdpreview_prepare(CpInfo *cpinfo)
       continue;
     }
 
-    CpBufInfo cp_bufinfo;
-    cp_bufinfo.buf = buf;
-
-    cp_bufinfo.save_b_p_ul = buf->b_p_ul;
-    cp_bufinfo.save_b_changed = buf->b_changed;
-    cp_bufinfo.save_changedtick = buf_get_changedtick(buf);
-
-    cmdpreview_save_undo(&cp_bufinfo.undo_info, buf);
-    u_clearall(buf);
-
-    kv_push(cpinfo->buf_info, cp_bufinfo);
+    if (!set_has(ptr_t, &saved_bufs, buf)) {
+      CpBufInfo cp_bufinfo;
+      cp_bufinfo.buf = buf;
+      cp_bufinfo.save_b_p_ul = buf->b_p_ul;
+      cp_bufinfo.save_b_changed = buf->b_changed;
+      cp_bufinfo.save_changedtick = buf_get_changedtick(buf);
+      cmdpreview_save_undo(&cp_bufinfo.undo_info, buf);
+      kv_push(cpinfo->buf_info, cp_bufinfo);
+      set_put(ptr_t, &saved_bufs, buf);
 
-    buf->b_p_ul = LONG_MAX;     // Make sure we can undo all changes
+      u_clearall(buf);
+      buf->b_p_ul = LONG_MAX;  // Make sure we can undo all changes
+    }
 
     CpWinInfo cp_wininfo;
     cp_wininfo.win = win;
@@ -2375,6 +2377,8 @@ static void cmdpreview_prepare(CpInfo *cpinfo)
     win->w_p_cuc = false;       // Disable 'cursorcolumn' so it doesn't mess up the highlights
   }
 
+  set_destroy(ptr_t, &saved_bufs);
+
   cpinfo->save_hls = p_hls;
   cpinfo->save_cmdmod = cmdmod;
   win_size_save(&cpinfo->save_view);
@@ -2431,6 +2435,7 @@ static void cmdpreview_restore_state(CpInfo *cpinfo)
     // Clear preview highlights.
     extmark_clear(buf, (uint32_t)cmdpreview_ns, 0, 0, MAXLNUM, MAXCOL);
   }
+
   for (size_t i = 0; i < cpinfo->win_info.size; i++) {
     CpWinInfo cp_wininfo = cpinfo->win_info.items[i];
     win_T *win = cp_wininfo.win;
diff --git a/test/functional/ui/inccommand_spec.lua b/test/functional/ui/inccommand_spec.lua
index dc4aeb9c83..00dfa3fd49 100644
--- a/test/functional/ui/inccommand_spec.lua
+++ b/test/functional/ui/inccommand_spec.lua
@@ -174,9 +174,12 @@ describe(":substitute, 'inccommand' preserves", function()
     it("'undolevels' (inccommand="..case..")", function()
       feed_command("set undolevels=139")
       feed_command("setlocal undolevels=34")
+      feed_command("split")  -- Show the buffer in multiple windows
       feed_command("set inccommand=" .. case)
       insert("as")
-      feed(":%s/as/glork/<enter>")
+      feed(":%s/as/glork/")
+      poke_eventloop()
+      feed("<enter>")
       eq(meths.get_option_value('undolevels', {scope='global'}), 139)
       eq(meths.get_option_value('undolevels', {buf=0}), 34)
     end)