Fix edge case with multiline lists with only a comment and no items

src/runestone.jl

@@ -2485,13 +2485,16 @@ function indent_listlike(
     end
     # Kid just before the closing token should be a newline and it should be tagged with
     # pre-dedent.
-    if close_idx - 1 == open_idx + 1
+    if idx_after_leading_nl == close_idx
         # Just a single kid which should then have both leading and trailing newline
         if any_kid_changed
-            # Modify this kid again by popping from the list and backtrack the stream
+            # Modify this kid again by popping from the list
             kid = pop!(kids′)
-            seek(ctx.fmt_io, position(ctx.fmt_io) - span(kid))
+        else
+            kid = kids[close_idx - 1]
         end
+        # Backtrack the stream
+        seek(ctx.fmt_io, position(ctx.fmt_io) - span(kid))
     else
         kid = kids[close_idx - 1]
     end

test/runtests.jl

@@ -397,6 +397,9 @@ end
     # Non space whitespace (TODO: Not sure if a JuliaSyntax bug or not?)
     @test format_string(String(UInt8[0x61, 0x20, 0x3d, 0x3d, 0x20, 0xc2, 0xa0, 0x62, 0x3a, 0x63])) ==
         "a == b:c"
+    # Edge case with comment and no items
+    @test format_string("[# a\n]") == "[# a\n]"
+    @test format_string("[ # a\n]") == "[ # a\n]"
 end
 
 @testset "whitespace around ->" begin