Gnucash attempts to set up en_US-like defaults in the C locale, but
assumes the "grouping" and "mon_grouping" values will be set to the
empty string in that locale.  Illumos insteads sets the grouping
values to "\177\0" (which is a different way to express "no grouping".

The report regression tests explicitly set the "C" locale and assume
you'll get 3-digit grouping with commas in the C locale; with this
patch, all of the regression tests now pass.

--- gnucash-5.11/libgnucash/core-utils/gnc-locale-utils.c.~1~	Mon Nov 11 18:02:45 2024
+++ gnucash-5.11/libgnucash/core-utils/gnc-locale-utils.c	Thu May  1 18:25:20 2025
@@ -65,7 +65,25 @@
     }
 }
 
+/*
+ * Force grouping if there is a separator but no grouping is set.
+ * Not ideal but seems to be necessary to get regression tests to
+ * pass on illumos (espcially since libstdc++ kills us in a non-C locale)
+ */
 static void
+gnc_lconv_set_grouping (char **p_value, char *default_value, char *sep)
+{
+    char *value = *p_value;
+
+    if ((value == NULL) || (value[0] == 0)) {
+	*p_value = default_value;
+	return;
+    }
+    if ((value[0] == CHAR_MAX) && sep != NULL && sep[0] != 0)
+	*p_value = default_value;
+}
+
+static void
 gnc_lconv_set_char (char *p_value, char default_value)
 {
     if ((p_value != NULL) && (*p_value == CHAR_MAX))
@@ -85,12 +103,12 @@
 
     gnc_lconv_set_utf8(&lc.decimal_point, ".");
     gnc_lconv_set_utf8(&lc.thousands_sep, ",");
-    gnc_lconv_set_utf8(&lc.grouping, "\003");
+    gnc_lconv_set_grouping(&lc.grouping, "\003", lc.thousands_sep);
     gnc_lconv_set_utf8(&lc.int_curr_symbol, "USD ");
     gnc_lconv_set_utf8(&lc.currency_symbol, "$");
     gnc_lconv_set_utf8(&lc.mon_decimal_point, ".");
     gnc_lconv_set_utf8(&lc.mon_thousands_sep, ",");
-    gnc_lconv_set_utf8(&lc.mon_grouping, "\003");
+    gnc_lconv_set_grouping(&lc.mon_grouping, "\003", lc.mon_thousands_sep);
     gnc_lconv_set_utf8(&lc.negative_sign, "-");
     gnc_lconv_set_utf8(&lc.positive_sign, "");