fix(preview): keep rendered lines inside the window

lua/md-render/preview.lua

@@ -29,6 +29,18 @@ local function usable_win_width(win)
   return math.max(1, total - textoff)
 end
 
+--- The `max_width` to render at for a window.  ContentBuilder wraps the text
+--- at `max_width` and then prepends `indent`, so the indent has to come off
+--- the window width; otherwise lines run up to its width past the edge and
+--- 'wrap' folds their last characters onto a row of their own.
+---@param win integer
+---@param indent? string defaults to ContentBuilder's `"  "`
+---@return integer
+local function content_width(win, indent)
+  local indent_w = vim.api.nvim_strwidth(indent or "  ")
+  return math.max(1, math.min(usable_win_width(win) - indent_w, DEFAULT_MAX_WIDTH))
+end
+
 --- Parse simple YAML frontmatter lines into key-value pairs
 ---@param fm_lines string[]
 ---@return {key: string, value: string}[]
@@ -503,7 +515,7 @@ end
 function Session:bind_window(win)
   self.win = win
   if not self._explicit_max_width then
-    local win_width = math.min(usable_win_width(win), DEFAULT_MAX_WIDTH)
+    local win_width = content_width(win, self.opts.indent)
     if win_width ~= (self.opts.max_width or DEFAULT_MAX_WIDTH) then
       self.opts.max_width = win_width
       self:rebuild()
@@ -1752,7 +1764,7 @@ local function install_win_resize_handler(session)
 
       local win = render_wins[1]
       if not vim.api.nvim_win_is_valid(win) then return end
-      local win_width = math.min(usable_win_width(win), DEFAULT_MAX_WIDTH)
+      local win_width = content_width(win, session.opts.indent)
       if win_width == (session.opts.max_width or DEFAULT_MAX_WIDTH) then return end
 
       session.opts.max_width = win_width

tests/content_width_test.lua

@@ -0,0 +1,97 @@
+-- Test that rendered lines fit the window they are shown in.  ContentBuilder
+-- wraps at `max_width` and prepends the indent afterwards, so a window-sized
+-- `max_width` let lines run past the edge by the indent's width, and 'wrap'
+-- put their last characters on a screen row of their own at column 0.
+-- Run: nvim --headless -u NONE --noplugin -l tests/content_width_test.lua
+
+package.path = vim.fn.getcwd() .. "/lua/?.lua;" .. vim.fn.getcwd() .. "/lua/?/init.lua;" .. package.path
+
+local preview = require "md-render.preview"
+
+local pass_count = 0
+local fail_count = 0
+
+local function assert_eq(actual, expected, msg)
+  if vim.deep_equal(actual, expected) then
+    pass_count = pass_count + 1
+  else
+    fail_count = fail_count + 1
+    print("FAIL: " .. msg)
+    print("  expected: " .. vim.inspect(expected))
+    print("  actual:   " .. vim.inspect(actual))
+  end
+end
+
+local function test(name, fn)
+  local ok, err = pcall(fn)
+  if not ok then
+    fail_count = fail_count + 1
+    print("ERROR: " .. name .. ": " .. tostring(err))
+  end
+end
+
+-- Mixed CJK and ASCII so that the wrap points land at odd and even columns.
+local DOC = {
+  "# Title",
+  "",
+  "- [x] " .. string.rep("あいうえお abc ", 8),
+  "    - " .. string.rep("かきくけこ `x = 1` ", 8),
+  "",
+  string.rep("さしすせそ def ", 10),
+}
+
+local function setup_md_buffer()
+  vim.cmd "silent! only"
+  local buf = vim.api.nvim_create_buf(false, false)
+  vim.bo[buf].filetype = "markdown"
+  vim.api.nvim_buf_set_name(buf, "/tmp/md-render-content-width-test-" .. buf .. ".md")
+  vim.api.nvim_buf_set_lines(buf, 0, -1, false, DOC)
+  vim.api.nvim_win_set_buf(0, buf)
+  return buf
+end
+
+--- Lines of the window's buffer wider than its text area.
+local function overflowing(win)
+  local width = vim.api.nvim_win_get_width(win) - vim.fn.getwininfo(win)[1].textoff
+  local out = {}
+  for _, line in ipairs(vim.api.nvim_buf_get_lines(vim.api.nvim_win_get_buf(win), 0, -1, false)) do
+    if vim.api.nvim_strwidth(line) > width then table.insert(out, line) end
+  end
+  return out
+end
+
+local function render_win()
+  for _, w in ipairs(vim.api.nvim_tabpage_list_wins(0)) do
+    if vim.bo[vim.api.nvim_win_get_buf(w)].filetype == "md-render" then return w end
+  end
+end
+
+for _, columns in ipairs { 40, 51, 60, 73 } do
+  test("toggle at " .. columns .. " columns", function()
+    vim.o.columns = columns
+    local source = setup_md_buffer()
+    preview.toggle()
+    assert_eq(overflowing(vim.api.nvim_get_current_win()), {}, "toggle lines fit at " .. columns .. " columns")
+    preview.toggle()
+    pcall(vim.api.nvim_buf_delete, source, { force = true })
+  end)
+
+  test("split at " .. columns .. " columns", function()
+    vim.o.columns = columns
+    local source = setup_md_buffer()
+    preview.split { mods = { vertical = false } }
+    assert_eq(overflowing(render_win()), {}, "split lines fit at " .. columns .. " columns")
+    pcall(vim.api.nvim_buf_delete, source, { force = true })
+  end)
+
+  test("pager at " .. columns .. " columns", function()
+    vim.o.columns = columns
+    local source = setup_md_buffer()
+    preview.show_pager()
+    assert_eq(overflowing(render_win()), {}, "pager lines fit at " .. columns .. " columns")
+    pcall(vim.api.nvim_buf_delete, source, { force = true })
+  end)
+end
+
+print(string.format("content_width_test: %d passed, %d failed", pass_count, fail_count))
+if fail_count > 0 then os.exit(1) end