4 files changed, 58 insertions, 7 deletions

diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index 8d82d22040..7c381ac1ed 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -1400,6 +1400,9 @@ Array nvim_buf_get_extmarks(Buffer buffer, Integer ns_id,
 ///                   callbacks. The mark will only be used for the current
 ///                   redraw cycle, and not be permantently stored in the
 ///                   buffer.
+///               - gravity : the direction the extmark will be shifted in
+///                   when new text is inserted. Must be either 'left' or
+///                   'right'.
 /// @param[out]  err   Error details, if any
 /// @return Id of the created/updated extmark
 Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id,
@@ -1440,6 +1443,7 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id,
   DecorPriority priority = DECOR_PRIORITY_BASE;
   colnr_T col2 = 0;
   VirtText virt_text = KV_INITIAL_VALUE;
+  bool right_gravity = true;
   for (size_t i = 0; i < opts.size; i++) {
     String k = opts.items[i].key;
     Object *v = &opts.items[i].value;
@@ -1522,6 +1526,19 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id,
         goto error;
       }
       priority = (DecorPriority)v->data.integer;
+    } else if (strequal("gravity", k.data)) {
+      if (v->type != kObjectTypeString) {
+        api_set_error(err, kErrorTypeValidation,
+                      "gravity must be a string");
+        goto error;
+      }
+      if (strequal("left", v->data.string.data)) {
+        right_gravity = false;
+      } else if (!strequal("right", v->data.string.data)) {
+        api_set_error(err, kErrorTypeValidation,
+                      "invalid option for gravity: must be 'left' or 'right'");
+        goto error;
+      }
     } else {
       api_set_error(err, kErrorTypeValidation, "unexpected key: %s", k.data);
       goto error;
@@ -1572,7 +1589,7 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id,
     }
 
     id = extmark_set(buf, (uint64_t)ns_id, id, (int)line, (colnr_T)col,
-                     line2, col2, decor, kExtmarkNoUndo);
+                     line2, col2, decor, right_gravity, kExtmarkNoUndo);
   }
 
   return (Integer)id;
@@ -1687,7 +1704,7 @@ Integer nvim_buf_add_highlight(Buffer buffer,
   extmark_set(buf, ns_id, 0,
               (int)line, (colnr_T)col_start,
               end_line, (colnr_T)col_end,
-              decor_hl(hl_id), kExtmarkNoUndo);
+              decor_hl(hl_id), true, kExtmarkNoUndo);
   return src_id;
 }
 
@@ -1796,7 +1813,7 @@ Integer nvim_buf_set_virtual_text(Buffer buffer,
   Decoration *decor = xcalloc(1, sizeof(*decor));
   decor->virt_text = virt_text;
 
-  extmark_set(buf, ns_id, 0, (int)line, 0, -1, -1, decor, kExtmarkNoUndo);
+  extmark_set(buf, ns_id, 0, (int)line, 0, -1, -1, decor, true, kExtmarkNoUndo);
   return src_id;
 }
 
diff --git a/src/nvim/decoration.c b/src/nvim/decoration.c
index e6a616c927..0a096bec9e 100644
--- a/src/nvim/decoration.c
+++ b/src/nvim/decoration.c
@@ -69,7 +69,7 @@ void bufhl_add_hl_pos_offset(buf_T *buf,
     }
     (void)extmark_set(buf, (uint64_t)src_id, 0,
                       (int)lnum-1, hl_start, (int)lnum-1+end_off, hl_end,
-                      decor, kExtmarkNoUndo);
+                      decor, true, kExtmarkNoUndo);
   }
 }
 
diff --git a/src/nvim/extmark.c b/src/nvim/extmark.c
index b2d8532cd7..3f3b8c76f3 100644
--- a/src/nvim/extmark.c
+++ b/src/nvim/extmark.c
@@ -71,7 +71,7 @@ static ExtmarkNs *buf_ns_ref(buf_T *buf, uint64_t ns_id, bool put) {
 /// @returns the mark id
 uint64_t extmark_set(buf_T *buf, uint64_t ns_id, uint64_t id,
                      int row, colnr_T col, int end_row, colnr_T end_col,
-                     Decoration *decor, ExtmarkOp op)
+                     Decoration *decor, bool right_gravity, ExtmarkOp op)
 {
   ExtmarkNs *ns = buf_ns_ref(buf, ns_id, true);
   assert(ns != NULL);
@@ -109,10 +109,10 @@ uint64_t extmark_set(buf_T *buf, uint64_t ns_id, uint64_t id,
 
   if (end_row > -1) {
     mark = marktree_put_pair(buf->b_marktree,
-                             row, col, true,
+                             row, col, right_gravity,
                              end_row, end_col, false);
   } else {
-    mark = marktree_put(buf->b_marktree, row, col, true);
+    mark = marktree_put(buf->b_marktree, row, col, right_gravity);
   }
 
 revised:
diff --git a/test/functional/api/extmark_spec.lua b/test/functional/api/extmark_spec.lua
index ab913ba4a4..159b8f7a58 100644
--- a/test/functional/api/extmark_spec.lua
+++ b/test/functional/api/extmark_spec.lua
@@ -1389,6 +1389,40 @@ describe('API/extmarks', function()
       undo]],false)
     eq(2, meths.eval('1+1')) -- did not crash
   end)
+
+  it('works with left and right gravity', function()
+    -- right gravity should move with inserted text, while
+    -- left gravity should stay in place.
+    curbufmeths.set_extmark(ns, 0, 5, {gravity = 'left'})
+    curbufmeths.set_extmark(ns, 0, 5, {gravity = 'right'})
+    feed([[Aasdfasdf]])
+
+    eq({ {1, 0, 5}, {2, 0, 13} },
+        curbufmeths.get_extmarks(ns, 0, -1, {}))
+
+    -- but both move when text is inserted before
+    feed([[<esc>Iasdf<esc>]])
+    -- eq({}, curbufmeths.get_lines(0, -1, true))
+    eq({ {1, 0, 9}, {2, 0, 17} },
+        curbufmeths.get_extmarks(ns, 0, -1, {}))
+
+    -- clear text
+    curbufmeths.set_text(0, 0, 0, 17, {})
+
+    -- handles set_text correctly as well
+    eq({ {1, 0, 0}, {2, 0, 0} },
+        meths.buf_get_extmarks(0, ns, 0, -1, {}))
+    curbufmeths.set_text(0, 0, 0, 0, {'asdfasdf'})
+    eq({ {1, 0, 0}, {2, 0, 8} },
+        curbufmeths.get_extmarks(ns, 0, -1, {}))
+
+    feed('u')
+    -- handles pasting
+    meths.exec([[let @a='asdfasdf']], false)
+    feed([["ap]])
+    eq({ {1, 0, 0}, {2, 0, 8} },
+        meths.buf_get_extmarks(0, ns, 0, -1, {}))
+  end)
 end)
 
 describe('Extmarks buffer api with many marks', function()