fix(lsp): start incremental sync range at previous newline character (#17610)

This change forces the start of an incremental sync range to begin always on an existing line.

runtime/lua/vim/lsp/sync.lua

@@ -131,13 +131,22 @@ end
 ---@param offset_encoding string utf-8|utf-16|utf-32|nil (fallback to utf-8)
 ---@returns table<int, int> line_idx, byte_idx, and char_idx of first change position
 local function compute_start_range(prev_lines, curr_lines, firstline, lastline, new_lastline, offset_encoding)
+  local char_idx
+  local byte_idx
   -- If firstline == lastline, no existing text is changed. All edit operations
   -- occur on a new line pointed to by lastline. This occurs during insertion of
   -- new lines(O), the new newline is inserted at the line indicated by
   -- new_lastline.
+  if firstline == lastline then
+    local line = prev_lines[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 }
+  end
+
   -- If firstline == new_lastline, the first change occurred on a line that was deleted.
   -- In this case, the first byte change is also at the first byte of firstline
-  if firstline == new_lastline or firstline == lastline then
+  if firstline == new_lastline then
     return { line_idx = firstline, byte_idx = 1, char_idx = 1 }
   end
 
@@ -158,8 +167,6 @@ local function compute_start_range(prev_lines, curr_lines, firstline, lastline,
   end
 
   -- Convert byte to codepoint if applicable
-  local char_idx
-  local byte_idx
   if start_byte_idx == 1 or (#prev_line == 0 and start_byte_idx == 1)then
     byte_idx = start_byte_idx
     char_idx = 1

test/functional/plugin/lsp/incremental_sync_spec.lua

@@ -207,16 +207,16 @@ describe('incremental synchronization', function()
         {
           range = {
             ['start'] = {
-              character = 0,
-              line = 1
+              character = 11,
+              line = 0,
             },
             ['end'] = {
               character = 0,
               line = 1
             }
           },
-          rangeLength = 0,
-          text = 'hello world\n'
+          rangeLength = 1,
+          text = '\nhello world\n'
         }
       }
       test_edit({"hello world"}, {"yyp"}, expected_text_changes, 'utf-16', '\n')
@@ -226,20 +226,39 @@ describe('incremental synchronization', function()
         {
           range = {
             ['start'] = {
-              character = 0,
-              line = 1
+              character = 11,
+              line = 0
             },
             ['end'] = {
               character = 0,
               line = 1
             }
           },
-          rangeLength = 0,
-          text = '\n'
+          rangeLength = 1,
+          text = '\n\n'
         }
       }
       test_edit({"hello world"}, {"o"}, expected_text_changes, 'utf-16', '\n')
     end)
+    it('adding a line to an empty buffer', function()
+      local expected_text_changes = {
+        {
+          range = {
+            ['start'] = {
+              character = 0,
+              line = 0
+            },
+            ['end'] = {
+              character = 0,
+              line = 1
+            }
+          },
+          rangeLength = 1,
+          text = '\n\n'
+        }
+      }
+      test_edit({""}, {"o"}, expected_text_changes, 'utf-16', '\n')
+    end)
   end)
   describe('multi line edit', function()
     it('deletion and insertion', function()