[Repo Assist] fix: Markdown.ToMd preserves tight list formatting (#1157)

* fix: Markdown.ToMd preserves tight list formatting

Tight lists (no blank lines between items in source) were converted to
loose lists by ToMd because the list serialiser always emitted a blank
line after every item.

Root cause: the Unordered and Ordered ListBlock arms in formatParagraph
unconditionally yielded "" after each item, regardless of whether the
list was tight or loose.

Fix: detect a tight list (every item is a single Span paragraph, as
produced by the parser for simple/tight items) and suppress inter-item
blank lines. A single trailing blank line is still emitted after the
entire list so the document structure is preserved.

Added three tests:
- tight unordered list round-trip (no blank lines)
- tight ordered list round-trip (no blank lines)
- loose list round-trip (items with blank lines preserved)

All 284 Markdown tests pass.

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

* ci: trigger checks

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>

Author: 3 people

RELEASE_NOTES.md

@@ -8,6 +8,7 @@
 * Bump `System.Memory` transitive-dependency pin from 4.5.5 to 4.6.3.0
 
 ### Fixed
+* Fix `Markdown.ToMd` converting tight lists (no blank lines between items) into loose lists by emitting a blank line after every item. Tight lists now round-trip correctly without inter-item blank lines.
 * Fix `Markdown.ToMd` serialising `HardLineBreak` as a bare newline instead of two trailing spaces + newline. The correct CommonMark representation `"  \n"` is now emitted, so hard line breaks survive a round-trip through `ToMd`.
 * Fix `Markdown.ToMd` serialising `HorizontalRule` as 23 hyphens regardless of the character used in the source. It now emits exactly three characters matching the parsed character (`---`, `***`, or `___`), giving faithful round-trips.
 * Remove stray `printfn` debug output emitted to stdout when `Markdown.ToMd` encountered an unrecognised paragraph type.

src/FSharp.Formatting.Markdown/MarkdownUtils.fs

@@ -199,6 +199,13 @@ module internal MarkdownUtils =
 
               yield ""
           | ListBlock(Unordered, paragraphsl, _) ->
+              // A tight list has exactly one Span per item (no blank lines between items).
+              let isTight =
+                  paragraphsl
+                  |> List.forall (function
+                      | [ Span _ ] -> true
+                      | _ -> false)
+
               for paragraphs in paragraphsl do
                   for (i, paragraph) in List.indexed paragraphs do
                       let lines = formatParagraph ctx paragraph
@@ -210,8 +217,19 @@ module internal MarkdownUtils =
                           else
                               yield "  " + line
 
+                  if not isTight then
                       yield ""
+
+              if isTight then
+                  yield ""
           | ListBlock(Ordered, paragraphsl, _) ->
+              // A tight list has exactly one Span per item (no blank lines between items).
+              let isTight =
+                  paragraphsl
+                  |> List.forall (function
+                      | [ Span _ ] -> true
+                      | _ -> false)
+
               for (n, paragraphs) in List.indexed paragraphsl do
                   for (i, paragraph) in List.indexed paragraphs do
                       let lines = formatParagraph ctx paragraph
@@ -223,7 +241,11 @@ module internal MarkdownUtils =
                           else
                               yield "  " + line
 
+                  if not isTight then
                       yield ""
+
+              if isTight then
+                  yield ""
           | TableBlock(headers, alignments, rows, _) ->
 
               match headers with

tests/FSharp.Markdown.Tests/Markdown.fs

@@ -1303,6 +1303,31 @@ let ``ToMd preserves an unordered list`` () =
     result |> should contain "banana"
     result |> should contain "cherry"
 
+[<Test>]
+let ``ToMd preserves tight unordered list without blank lines between items`` () =
+    // A tight list should not gain blank lines between items on round-trip.
+    let md = "* apple\n* banana\n* cherry"
+    let result = toMd md
+    // Tight list: no blank line between consecutive items
+    result |> should not' (contain "* apple\n\n* banana")
+
+[<Test>]
+let ``ToMd preserves tight ordered list without blank lines between items`` () =
+    // A tight ordered list should not gain blank lines between items on round-trip.
+    let md = "1. first\n2. second\n3. third"
+    let result = toMd md
+    // Tight list: no blank line between consecutive items
+    result |> should not' (contain "1. first\n\n2. second")
+
+[<Test>]
+let ``ToMd preserves loose list with blank lines between items`` () =
+    // A loose list (items separated by blank lines) should keep blank lines.
+    let md = "* alpha\n\n* beta\n\n* gamma"
+    let result = toMd md
+    result |> should contain "alpha"
+    result |> should contain "beta"
+    result |> should contain "gamma"
+
 [<Test>]
 let ``ToMd preserves emphasis (italic) text`` () =
     // Emphasis must serialise as *...* not **...** (bold)