Fix a bug in computing table alignments.

When there are colspans in the header, alignments weren't computed
correctly.   With this patch, if a header cell with colspans has
an alignment, that alignment is given to all the columns spanned.

Resolves jgm/pandoc#9085.

Found with the help of Claude Opus 5.5.

Author: jgm

src/Text/GridTable/Trace.hs

@@ -27,7 +27,7 @@ import Data.Array.MArray
 import Data.Array.ST
 import Data.Function (on)
 import Data.List (foldl')
-import Data.Maybe (fromMaybe)
+import Data.Maybe (fromMaybe, listToMaybe)
 import Data.Set (Set)
 import Data.Text (Text)
 import Text.DocLayout (charWidth, realLength)
@@ -412,13 +412,18 @@ tableFromTraceInfo traceInfo partSeps colSpecsFirstLine =
       rowindex = Map.fromList $ zip rowseps [1..]
       colindex = Map.fromList $ zip colseps [1..]
       colwidths = [ b - a - 1 | (b, a) <- zip (drop 1 colseps) colseps ]
+      alignSpecs = case partSeps of
+                     partSep:_ -> partSepColSpec partSep
+                     []        -> fromMaybe [] colSpecsFirstLine
+      -- A segment of the alignment line may span multiple columns;
+      -- each column gets the alignment of the segment containing it.
+      alignFor (left, right) = fromMaybe AlignDefault $ listToMaybe
+        [ colAlign cs | cs <- alignSpecs
+                      , colStart cs - 1 <= left
+                      , right <= colEnd cs ]
       colSpecs = zip
-                 (map colAlign
-                   (case partSeps of
-                       partSep:_ -> partSepColSpec partSep
-                       []        -> fromMaybe [] colSpecsFirstLine)
-                   ++ repeat AlignDefault)
-                 (map fromCharCol colwidths)
+                  (map alignFor (zip colseps (drop 1 colseps)))
+                  (map fromCharCol colwidths)
       lastCol = ColIndex (length colwidths)
       mlastLine = Set.lookupMax (gridRowSeps traceInfo)
       tableHead = case partSeps of

test/test-gridtables.hs

@@ -528,6 +528,42 @@ gridTableTests = testGroup "parseArrayTable"
                      , [Cell "3" 1 1]
                      ]
     ]
+  , testGroup "markers spanning multiple columns"
+    [ testCase "marker in first line" $
+      let gt = T.unlines
+               [ "+:------------:+---------------:+"
+               , "| Heading 1    | Heading 2      |"
+               , "+---+----+-----+----+------+----+"
+               , "| A | B  | C   | D  |  E   | F  |"
+               , "+---+----+-----+----+------+----+"
+               ]
+      in arrayTableColSpecs <$> parse' gridTable gt @?=
+         Right (listArray (1, 6)
+                [ (AlignCenter, 3)
+                , (AlignCenter, 4)
+                , (AlignCenter, 5)
+                , (AlignRight, 4)
+                , (AlignRight, 6)
+                , (AlignRight, 4)
+                ])
+
+    , testCase "marker in part separator" $
+      let gt = T.unlines
+               [ "+-------------+-------+"
+               , "| head        | c     |"
+               , "+:===========:+======:+"
+               , "| ab          | c     |"
+               , "+------+------+-------+"
+               , "| a    | b    | c     |"
+               , "+------+------+-------+"
+               ]
+      in arrayTableColSpecs <$> parse' gridTable gt @?=
+         Right (listArray (1, 3)
+                [ (AlignCenter, 6)
+                , (AlignCenter, 6)
+                , (AlignRight, 7)
+                ])
+    ]
   ]
 
 defaultAlign :: [Int] -> Array ColIndex (Alignment, Int)