fix(lsp): handle insertion of previous line (#17618)

runtime/lua/vim/lsp/sync.lua

@@ -138,10 +138,18 @@ local function compute_start_range(prev_lines, curr_lines, firstline, lastline,
   -- new lines(O), the new newline is inserted at the line indicated by
   -- new_lastline.
   if firstline == lastline then
+    local line_idx
     local line = prev_lines[firstline - 1]
+    if line then
+      line_idx = firstline - 1
       byte_idx = #line + 1
       char_idx = compute_line_length(line, offset_encoding) + 1
-    return { line_idx = firstline - 1, byte_idx = byte_idx, char_idx = char_idx }
+    else
+      line_idx = firstline
+      byte_idx = 1
+      char_idx = 1
+    end
+    return { line_idx = line_idx, byte_idx = byte_idx, char_idx = char_idx }
   end
 
   -- If firstline == new_lastline, the first change occurred on a line that was deleted.

test/functional/plugin/lsp/incremental_sync_spec.lua

@@ -259,6 +259,25 @@ describe('incremental synchronization', function()
       }
       test_edit({""}, {"o"}, expected_text_changes, 'utf-16', '\n')
     end)
+    it('insert a line above the current line', function()
+      local expected_text_changes = {
+        {
+          range = {
+            ['start'] = {
+              character = 0,
+              line = 0
+            },
+            ['end'] = {
+              character = 0,
+              line = 0
+            }
+          },
+          rangeLength = 0,
+          text = '\n'
+        }
+      }
+      test_edit({""}, {"O"}, expected_text_changes, 'utf-16', '\n')
+    end)
   end)
   describe('multi line edit', function()
     it('deletion and insertion', function()