fix bug when extmark positions are inverted/beyond EOF

Author: jrop

lua/morph.lua

@@ -493,11 +493,10 @@ function Extmark._from_raw(bufnr, ns, id, start_row0, start_col0, details)
 
   -- Clamp extmark bounds to actual buffer size (extmarks can overshoot after deletions)
   local last_line_idx = math.max(0, vim.api.nvim_buf_line_count(bufnr) - 1)
-  if extmark.stop[1] > last_line_idx then
-    local last_line = vim.api.nvim_buf_get_lines(bufnr, last_line_idx, last_line_idx + 1, true)[1]
-      or ''
-    extmark.stop = Pos00.new(last_line_idx, #last_line)
-  end
+  local last_line = vim.api.nvim_buf_get_lines(bufnr, last_line_idx, last_line_idx + 1, true)[1]
+    or ''
+  if extmark.start[1] > last_line_idx then extmark.start = Pos00.new(last_line_idx, #last_line) end
+  if extmark.stop[1] > last_line_idx then extmark.stop = Pos00.new(last_line_idx, #last_line) end
 
   return extmark
 end
@@ -534,6 +533,10 @@ function Extmark:_text()
   local start, stop = self.start, self.stop
   if start == stop then return '' end
 
+  -- Handle inverted positions (start > stop), which can occur after buffer
+  -- deletions. Return empty string as there's no valid content to extract.
+  if start > stop then return '' end
+
   -- Handle edge case: if stop is at column 0, we need to include the newline
   -- from the previous line, which getregion doesn't handle well
   local needs_trailing_newline = false
@@ -1433,6 +1436,7 @@ Morph.Pos00 = Pos00
 if vim.env.NVIM_TEST then
   Morph._is_textlock = is_textlock
   Morph._levenshtein = levenshtein
+  Morph.Extmark = Extmark
 end
 
 return Morph

spec/morph_spec.lua

@@ -8,6 +8,7 @@ vim.env.NVIM_TEST = 'true'
 local Morph = require 'morph'
 local h = Morph.h
 local Pos00 = Morph.Pos00
+local Extmark = Morph.Extmark
 
 --------------------------------------------------------------------------------
 -- TEST HELPERS
@@ -1280,6 +1281,77 @@ describe('Morph', function()
         end)
       end)
     end)
+
+    it('handles inverted extmark positions gracefully', function()
+      -- This test verifies that extmark positions becoming inverted
+      -- (start > stop) is handled gracefully by returning empty string.
+      -- The error message was:
+      --   (morph.nvim:getregion:invalid-pos) { start, end } = { { 9, 4, 1 }, { 9, 3, 9 } }
+      -- Note how line 4 > line 3, which is inverted.
+      with_buf({ 'line1', 'line2', 'line3' }, function()
+        -- Create an Extmark directly with inverted positions (start > stop)
+        -- This simulates what happens when extmarks get corrupted
+        local extmark = {
+          bufnr = 0,
+          ns = vim.api.nvim_create_namespace 'test',
+          -- Inverted: start at row 2, stop at row 1 (0-based)
+          -- This is the bug condition
+          start = Pos00.new(2, 0),
+          stop = Pos00.new(1, 5),
+        }
+        setmetatable(extmark, { __index = Extmark })
+
+        -- Calling _text() with inverted positions should return empty string
+        local result = extmark:_text()
+
+        assert.are.equal(
+          '',
+          result,
+          'Extmark:_text() should handle inverted positions gracefully (return empty string), but got: '
+            .. vim.inspect(result)
+        )
+      end)
+    end)
+  end)
+
+  -- Bug: Deleting trailing blank line causes getregion invalid-pos error
+  -- Error: (morph.nvim:getregion:invalid-pos) { start, end } = { { 9, 4, 1 }, { 9, 3, 9 } }
+  --
+  -- Root cause: Extmark._from_raw clamps stop but NOT start.
+  -- When an extmark's start is past buffer end after deletion, we get start > stop.
+
+  it('clamps extmark start position to buffer bounds', function()
+    with_buf({}, function()
+      local ns = vim.api.nvim_create_namespace 'test_extmark_clamp'
+
+      -- Buffer: 4 lines (0-3)
+      vim.api.nvim_buf_set_lines(0, 0, -1, true, { 'line 0', 'line 1', 'line 2', '' })
+      assert.are.same(4, line_count())
+
+      -- Extmark at line 3 (the last line)
+      local ext_id = vim.api.nvim_buf_set_extmark(0, ns, 3, 0, { end_row = 3, end_col = 0 })
+
+      -- Delete the last line → buffer now has 3 lines (0-2)
+      vim.api.nvim_buf_set_lines(0, 3, 4, true, {})
+      assert.are.same(3, line_count())
+
+      -- Neovim returns extmark with stale position (line 3 doesn't exist)
+      local raw = vim.api.nvim_buf_get_extmark_by_id(0, ns, ext_id, { details = true })
+      local start_row, start_col, details = raw[1], raw[2], raw[3]
+
+      -- BUG: _from_raw clamps stop to buffer bounds but NOT start
+      local extmark = Extmark._from_raw(0, ns, ext_id, start_row, start_col, details)
+
+      -- start should be clamped to valid buffer line
+      local last_line_idx = line_count() - 1
+      assert.is_true(
+        extmark.start[1] <= last_line_idx,
+        ('extmark.start[1] (%d) should be <= last_line_idx (%d)'):format(
+          extmark.start[1],
+          last_line_idx
+        )
+      )
+    end)
   end)
 
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