fix(docs-html): misaligned tabs after conceal #20690

Problem:
`gen_help_html.lua` does not properly handle tab characters after
"concealed" text (tags, taglinks, codespans). This causes misaligned
layout in "old" (preformatted) docs.

For text like `*tag*`, |tag_link|, and `code_span`, Vim hides the "*",
"|", "`" characters, but Vim still counts those characters for "virtual
column" when a tab character follows it. So if you have a tag of say
6 characters long, those two concealed character would lead to the tab
character after it start at column 8. gen_help_html.lua doesn't account
for that which leads to formatting flaws in the generated output.

Solution:
Add two spaces after concealed nodes that are followed by a tab char.

scripts/gen_help_html.lua

@@ -390,6 +390,18 @@ local function visit_validate(root, level, lang_tree, opt, stats)
   end
 end
 
+-- Fix tab alignment issues caused by concealed characters like |, `, * in tags
+-- and code blocks.
+local function fix_tab_after_conceal(text, next_node_text)
+  -- Vim tabs take into account the two concealed characters even though they
+  -- are invisible, so we need to add back in the two spaces if this is
+  -- followed by a tab to make the tab alignment to match Vim's behavior.
+  if string.sub(next_node_text,1,1) == '\t' then
+    text = text .. '  '
+  end
+  return text
+end
+
 -- Generates HTML from node `root` recursively.
 local function visit_node(root, level, lang_tree, headings, opt, stats)
   level = level or 0
@@ -506,12 +518,20 @@ local function visit_node(root, level, lang_tree, headings, opt, stats)
     if ignored then
       return text
     end
-    return ('%s<a href="%s#%s">%s</a>'):format(ws(), helppage, url_encode(tagname), html_esc(tagname))
+    local s = ('%s<a href="%s#%s">%s</a>'):format(ws(), helppage, url_encode(tagname), html_esc(tagname))
+    if node_name == 'taglink' and opt.old then
+      s = fix_tab_after_conceal(s, node_text(root:next_sibling()))
+    end
+    return s
   elseif vim.tbl_contains({'codespan', 'keycode'}, node_name) then
     if root:has_error() then
       return text
     end
-    return ('%s<code>%s</code>'):format(ws(), trimmed)
+    local s = ('%s<code>%s</code>'):format(ws(), trimmed)
+    if node_name == 'codespan' and opt.old then
+      s = fix_tab_after_conceal(s, node_text(root:next_sibling()))
+    end
+    return s
   elseif node_name == 'argument' then
     return ('%s<code>{%s}</code>'):format(ws(), text)
   elseif node_name == 'codeblock' then
@@ -533,6 +553,10 @@ local function visit_node(root, level, lang_tree, headings, opt, stats)
     end
     local el = in_heading and 'span' or 'code'
     local s = ('%s<a name="%s"></a><%s class="%s">%s</%s>'):format(ws(), url_encode(tagname), el, cssclass, trimmed, el)
+    if opt.old then
+        s = fix_tab_after_conceal(s, node_text(root:next_sibling()))
+    end
+
     if in_heading and prev ~= 'tag' then
       -- Start the <span> container for tags in a heading.
       -- This makes "justify-content:space-between" right-align the tags.