Reorder spanner columns to body order before rendering

lt matches a spanner to the visual position of its first column, then spans
the next columns.length cells. When a spanner's columns were listed in an
order other than the final left-to-right body order (e.g. after lt_move(),
or when selected by a predicate formula that returns data-frame order), the
mismatched first column shifted the colspan and could silently drop later
spanners.

Reorder each explicit spanner's columns to match `visible` (the post-move
body order) in resolveSpec() before the header row is built. This makes
spanners order-insensitive and lets predicate selectors like
`columns = ~ endsWith(., "_time")` be used for spanners safely. Columns must
still be contiguous in the body.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: yihui

NEWS.md

@@ -4,6 +4,8 @@
 
 - `lt_label()` now also accepts a single named list or named character vector mapping column names to labels (e.g., `lt_label(x, c(mpg = "Miles/Gallon", cyl = "Cylinders"))`), which is convenient when labels are computed programmatically. Named arguments (e.g., `lt_label(x, mpg = "Miles/Gallon")`) continue to work.
 
+- `lt_spanner()` no longer requires its `columns` to be listed in the table's visual (left-to-right) order. The columns of an explicit spanner are now reordered to match the final body order (after any `lt_move()`) before rendering, so a spanner is drawn correctly regardless of the order in which its columns were listed. This also makes predicate selectors (e.g., `columns = ~ endsWith(., "_time")`) safe to use for spanners. The columns must still be contiguous in the table body.
+
 # CHANGES IN lt VERSION 0.4
 
 - `lt_format()` gained a `sig_digits` argument to format numbers to a fixed number of significant digits (e.g., `lt_format(x, ~col, sig_digits = 3)`), which is useful for columns spanning several orders of magnitude. It is mutually exclusive with `decimals`.

inst/www/lt.js

@@ -352,8 +352,16 @@
     // Styles: the style ops are consumed directly (css/class/columns/rows/test).
     const styles = ops.filter(op => op.type === "style");
 
-    // Auto-spanners: split column names on separator, group contiguous prefixes
-    let spanners = spec.spanners || [];
+    // Explicit spanners: reorder each spanner's columns to match the final
+    // (post-move) body order, so a spanner is recorded/rendered correctly
+    // regardless of the order in which its columns were listed. A spanner is
+    // matched to the visual position of its first column and then spans the
+    // next columns.length cells, so a first column that is not the leftmost in
+    // `visible` would otherwise misalign the header row.
+    let spanners = (spec.spanners || []).map(sp => {
+      const cols = sp.columns.slice().sort((a, b) => visible.indexOf(a) - visible.indexOf(b));
+      return { ...sp, columns: cols };
+    });
     if (spec.auto_span) {
       const sep = typeof spec.auto_span === "string" ? new RegExp(spec.auto_span) : /[._]/;
       spanners = [...spanners];

tests/test-ci/test-js.R

@@ -202,6 +202,25 @@ assert("column spanner renders colspan", {
   (matches(html, '.*colspan="2".*>AB</th>.*') %==% "")
 })
 
+assert("spanner columns are reordered to body order before rendering", {
+  # columns listed out of order (b before a) still render as one spanner
+  html = build(list(
+    data = list(a = 1, b = 2, c = 3),
+    spanners = list(list(label = "AB", columns = list("b", "a")))
+  ))
+  (matches(html, '.*colspan="2".*>AB</th>.*') %==% "")
+
+  # after a move, a spanner whose columns are listed in the original (pre-move)
+  # order must anchor to the moved first column, not be dropped
+  html = build(list(
+    data = list(x = 1, a = 2, b = 3),
+    ops = list(list(type = "move", columns = list("b", "a"), after = "x")),
+    # body order becomes x, b, a; list spanner cols in the pre-move order
+    spanners = list(list(label = "AB", columns = list("a", "b")))
+  ))
+  (matches(html, '.*colspan="2".*>AB</th>.*') %==% "")
+})
+
 assert("footnotes render in tfoot", {
   html = build(list(
     data = list(x = 1),