fix(lists): expand tabs in the indentation to four-column stops

lua/md-render/content_builder.lua

@@ -1116,6 +1116,38 @@ local function list_content_column(line)
   return #ws + #marker + (#gap <= 4 and #gap or 1)
 end
 
+--- Expand tabs in each line's leading whitespace to spaces, with tab stops
+--- every four columns as CommonMark specifies.
+---
+--- Everything that measures indentation counts spaces, so a list item
+--- indented with a tab was one column deep instead of four: its continuation
+--- lines hung from the wrong column, and the tab itself reached the buffer,
+--- where 'tabstop' decided how wide it was.  Fenced code is left alone, since
+--- its tabs are content.
+---@param lines string[]
+---@return string[]
+local function expand_leading_tabs(lines)
+  local result = {}
+  local in_code = false
+  for i, line in ipairs(lines) do
+    result[i] = line
+    local fence = line:match "^%s*```" or line:match "^%s*~~~"
+    if fence then
+      in_code = not in_code
+    elseif not in_code then
+      local ws = line:match "^[ \t]*"
+      if ws:find("\t", 1, true) then
+        local col = 0
+        for c in ws:gmatch "." do
+          col = c == "\t" and (col + 4 - col % 4) or (col + 1)
+        end
+        result[i] = string.rep(" ", col) .. line:sub(#ws + 1)
+      end
+    end
+  end
+  return result
+end
+
 --- Move the content of a block container to column 0 and report the indent
 --- that was taken off each line.
 ---
@@ -1429,6 +1461,7 @@ function ContentBuilder:render_document(lines, opts)
   -- container_indents keeps the indent per *original* line so the loop below
   -- can restore it on output.
   local container_indents
+  lines = expand_leading_tabs(lines)
   lines, container_indents = strip_container_indent(lines)
   lines, src_indices = preprocess_multiline_html(lines, src_indices)
   lines, src_indices = join_paragraph_continuations(lines, src_indices, container_indents)

tests/list_indent_test.lua

@@ -153,5 +153,29 @@ do
   )
 end
 
+-- Test 10: a tab in the indentation advances to the next multiple of four,
+-- so a tab-indented item nests exactly as a four-space one does, wraps under
+-- its own text, and no tab reaches the output
+do
+  local long = string.rep("あ", 30)
+  local tabbed = render({ "- 外側", "\t- " .. long }, 24)
+  assert_eq(tabbed, render({ "- 外側", "    - " .. long }, 24), "a tab should indent like four spaces")
+  assert_eq(
+    #tabbed[3]:match "^ *",
+    vim.api.nvim_strwidth(tabbed[2]:match "^ *[^ ]+ "),
+    "a tab-indented item should hang under its text"
+  )
+  assert_eq(table.concat(tabbed):find("\t", 1, true), nil, "no tab should reach the output")
+
+  assert_eq(
+    render { "- 外側", "  \t- 内側" },
+    render { "- 外側", "    - 内側" },
+    "a tab after spaces should stop at column four"
+  )
+
+  local out = render { "- 項目", "", "  ```", "  \tx = 1", "  ```" }
+  assert_eq(out[2]:find("\t", 1, true) ~= nil, true, "a tab in fenced code is content and must stay")
+end
+
 print(string.format("\nlist_indent_test: %d passed, %d failed", pass_count, fail_count))
 if fail_count > 0 then os.exit(1) end