add some extmark-edge-case tests from morph.nvim

lua/u.lua

@@ -360,7 +360,7 @@ function Extmark.from_range(range, nsid)
   local r = range:to_charwise()
   local stop = r.stop or r.start
   local end_row = stop.lnum - 1
-  local end_col = stop.col
+  local end_col = r:is_empty() and (r.start.col - 1) or stop.col
   if range.mode == 'V' then
     end_row = end_row + 1
     end_col = 0
@@ -702,7 +702,7 @@ end
 function Range:to_charwise()
   local r = self:clone()
   r.mode = 'v'
-  if r.stop:is_col_max() then r.stop = r.stop:as_real() end
+  if r.stop and r.stop:is_col_max() then r.stop = r.stop:as_real() end
   return r
 end
 

spec/u_spec.lua

@@ -1370,6 +1370,61 @@ describe('Extmark additional coverage', function()
   end)
 end)
 
+describe('Extmark edge cases', function()
+  it('tracks multi-byte characters correctly', function()
+    withbuf({ '🚀🌟 hello 你好世界' }, function()
+      -- The string is 27 bytes: 🚀(4) + 🌟(4) + space(1) + hello(5) + space(1) + 你(3) + 好(3) + 世(3) + 界(3)
+      local r = Range.new(Pos.new(nil, 1, 1), Pos.new(nil, 1, 27), 'v')
+      local ext = r:save_to_extmark()
+      local ext_range = ext:range()
+      assert.are.same('🚀🌟 hello 你好世界', ext_range:text())
+    end)
+  end)
+
+  it('clamps start position after buffer shrink', function()
+    withbuf({ 'line 1', 'line 2', 'line 3', '' }, function()
+      local r = Range.from_buf_text()
+      local ext = r:save_to_extmark()
+
+      -- Delete last line
+      vim.api.nvim_buf_set_lines(0, 3, 4, true, {})
+
+      -- Get range from extmark - should clamp to valid buffer
+      local ext_range = ext:range()
+      assert.is_true(ext_range.stop.lnum <= 3)
+    end)
+  end)
+
+  it('handles zero-width extmark (empty range)', function()
+    withbuf({ 'hello world' }, function()
+      local r = Range.new(Pos.new(nil, 1, 1), nil, 'v')
+      local ext = r:save_to_extmark()
+      local ext_range = ext:range()
+      assert.is_true(ext_range:is_empty())
+    end)
+  end)
+
+  it('handles extmark at buffer start', function()
+    withbuf({ 'first', 'second', 'third' }, function()
+      local r = Range.new(Pos.new(nil, 1, 1), Pos.new(nil, 1, 5), 'v')
+      local ext = r:save_to_extmark()
+      local ext_range = ext:range()
+      assert.are.same(1, ext_range.start.lnum)
+      assert.are.same(1, ext_range.start.col)
+    end)
+  end)
+
+  it('handles extmark at buffer end', function()
+    withbuf({ 'first', 'second', 'third' }, function()
+      local r = Range.new(Pos.new(nil, 3, 1), Pos.new(nil, 3, 5), 'v')
+      local ext = r:save_to_extmark()
+      local ext_range = ext:range()
+      assert.are.same(3, ext_range.stop.lnum)
+      assert.are.same(5, ext_range.stop.col)
+    end)
+  end)
+end)
+
 describe('utils', function()
   local u = require 'u'