fix(decorations): crash with revised mark with changed decoration flags

fixes #27211

src/nvim/extmark.c

@@ -74,11 +74,15 @@ void extmark_set(buf_T *buf, uint32_t ns_id, uint32_t *idp, int row, colnr_T col
         if (old_mark.pos.row == row && old_mark.pos.col == col) {
           // not paired: we can revise in place
           if (!invalid && mt_decor_any(old_mark)) {
+            // TODO(bfredl): conflict of concerns: buf_decor_remove() must process
+            // the buffer as if MT_FLAG_DECOR_SIGNTEXT is already removed, however
+            // marktree must precisely adjust the set of flags from the old set to the new
+            uint16_t save_flags = mt_itr_rawkey(itr).flags;
             mt_itr_rawkey(itr).flags &= (uint16_t) ~MT_FLAG_DECOR_SIGNTEXT;
             buf_decor_remove(buf, row, row, col, mt_decor(old_mark), true);
+            mt_itr_rawkey(itr).flags = save_flags;
           }
-          mt_itr_rawkey(itr).flags &= (uint16_t) ~MT_FLAG_EXTERNAL_MASK;
-          mt_itr_rawkey(itr).flags |= flags;
+          marktree_revise_flags(buf->b_marktree, itr, flags);
           mt_itr_rawkey(itr).decor_data = decor.data;
           goto revised;
         }

src/nvim/marktree.c

@@ -774,6 +774,35 @@ uint64_t marktree_del_itr(MarkTree *b, MarkTreeIter *itr, bool rev)
   return other;
 }
 
+void marktree_revise_flags(MarkTree *b, MarkTreeIter *itr, uint16_t new_flags)
+{
+  uint32_t meta_old[4];
+  meta_describe_key(meta_old, rawkey(itr));
+  rawkey(itr).flags &= (uint16_t) ~MT_FLAG_EXTERNAL_MASK;
+  rawkey(itr).flags |= new_flags;
+
+  uint32_t meta_new[4];
+  meta_describe_key(meta_new, rawkey(itr));
+
+  if (!memcmp(meta_old, meta_new, sizeof(meta_old))) {
+    return;
+  }
+
+  MTNode *lnode = itr->x;
+  while (lnode->parent) {
+    uint32_t *meta_p = lnode->parent->meta[lnode->p_idx];
+    for (int m = 0; m < kMTMetaCount; m++) {
+      meta_p[m] += meta_new[m] - meta_old[m];
+    }
+
+    lnode = lnode->parent;
+  }
+
+  for (int m = 0; m < kMTMetaCount; m++) {
+    b->meta_root[m] += meta_new[m] - meta_old[m];
+  }
+}
+
 /// similar to intersect_common but modify x and y in place to retain
 /// only the items which are NOT in common
 static void intersect_merge(Intersection *restrict m, Intersection *restrict x,

test/functional/ui/decorations_spec.lua

@@ -2371,6 +2371,36 @@ describe('extmark decorations', function()
                                                         |
     ]]}
   end)
+
+  it('can replace marks in place with different decorations #27211', function()
+    local mark = api.nvim_buf_set_extmark(0, ns, 0, 0, { virt_lines = {{{"foo", "ErrorMsg"}}}, })
+    screen:expect{grid=[[
+      ^                                                  |
+      {4:foo}                                               |
+      {1:~                                                 }|*12
+                                                        |
+    ]]}
+
+    api.nvim_buf_set_extmark(0, ns, 0, 0, {
+      id = mark,
+      virt_text = { { "testing", "NonText" } },
+      virt_text_pos = "inline",
+    })
+    screen:expect{grid=[[
+      {1:^testing}                                           |
+      {1:~                                                 }|*13
+                                                        |
+    ]]}
+
+    api.nvim_buf_del_extmark(0, ns, mark)
+    screen:expect{grid=[[
+      ^                                                  |
+      {1:~                                                 }|*13
+                                                        |
+    ]]}
+
+    helpers.assert_alive()
+  end)
 end)
 
 describe('decorations: inline virtual text', function()