perf: avoid Seq allocations in hot markdown-parsing paths

- removeSpaces: replace Seq.takeWhile+Seq.length per line with
  String.TrimStart().Length — avoids boxing each char and allocating
  two enumerators per non-empty line.
- StartsWithNTimesTrimIgnoreStartWhitespace: replace Seq.windowed +
  Seq.map String + Seq.takeWhile + Seq.length with a direct index loop
  — avoids O(n) sliding-window allocations just to count consecutive
  fence characters (e.g. backticks or tildes) at the start of a line.
  Called for every line during markdown block parsing.
- XmlDocReader.readXmlElementAsSingleSummary: same Seq.takeWhile fix
  when checking indentation columns of XML doc comment lines.

All 520 tests pass (317 Markdown, 143 Literate, 30 CodeFormat, 30 ApiDocs).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

RELEASE_NOTES.md

@@ -3,6 +3,7 @@
 ## [Unreleased]
 
 ### Changed
+* Avoid `Seq` allocations in hot markdown-parsing paths. `removeSpaces` (called on every XML doc comment and literate code block) previously iterated each line character-by-character via `Seq.takeWhile + Seq.length`; it now uses `String.TrimStart().Length`. The `StartsWithNTimesTrimIgnoreStartWhitespace` active pattern (called for every line to detect fenced code blocks) previously built a sliding-window `Seq`, allocated a `System.String` per window, and counted matches via `Seq.length`; it now uses a simple index loop.
 * Compile `Regex` instances to module-level singletons (with `RegexOptions.Compiled`) in `PageContentList`, `HtmlFormatting`, `Formatting`, `Menu`, and `LlmsTxt`. Previously a new, uncompiled `Regex` was constructed on every call (once per page heading, once per HTML page, once per menu item, once per llms.txt entry), incurring repeated JIT overhead. The patterns are now compiled once at module load and reused across all calls.
 * Replace deprecated `System.Net.WebClient` with `System.Net.Http.HttpClient` in the image downloader used by `--saveimages`. Removes the `#nowarn "44"` suppression.
 * Bump `Newtonsoft.Json` transitive-dependency pin from 13.0.3 to 13.0.4.

src/Common/StringParsing.fs

@@ -110,7 +110,7 @@ module String =
             lines
             |> Seq.choose (fun line ->
                 if String.IsNullOrWhiteSpace line |> not then
-                    line |> Seq.takeWhile Char.IsWhiteSpace |> Seq.length |> Some
+                    line.Length - line.TrimStart().Length |> Some
                 else
                     None)
             |> fun xs -> if Seq.isEmpty xs then 0 else Seq.min xs
@@ -226,10 +226,16 @@ module StringPosition =
                 let startAndRest = text.Substring(beforeStart.Length)
 
                 let startNum =
-                    Seq.windowed start.Length startAndRest
-                    |> Seq.map (fun chars -> System.String(chars))
-                    |> Seq.takeWhile ((=) start)
-                    |> Seq.length
+                    let mutable count = 0
+                    let mutable pos = 0
+                    let startLen = start.Length
+
+                    while pos + startLen <= startAndRest.Length
+                          && startAndRest.Substring(pos, startLen) = start do
+                        count <- count + 1
+                        pos <- pos + startLen
+
+                    count
 
                 Some(
                     startNum,

src/FSharp.Formatting.ApiDocs/XmlDocReader.fs

@@ -133,7 +133,7 @@ module internal XmlDocReader =
         else
             let allLinesHaveSameColumn =
                 nonEmptyLines
-                |> Array.map (fun line -> line |> Seq.takeWhile (fun c -> c = ' ') |> Seq.length)
+                |> Array.map (fun line -> line.Length - line.TrimStart([| ' ' |]).Length)
                 |> Array.distinct
                 |> Array.length
                 |> (=) 1