Index: source/texk/web2c/luatexdir/tex/commands.w
===================================================================
--- source/texk/web2c/luatexdir/tex/commands.w	(revision 3950)
+++ source/texk/web2c/luatexdir/tex/commands.w	(working copy)
@@ -364,6 +364,9 @@
     primitive_luatex("Umathaccent", math_accent_cmd, 2, 0);
     primitive_luatex("Umathbotaccent", math_accent_cmd, 3, 0);
     primitive_luatex("Umathaccents", math_accent_cmd, 4, 0);
+    primitive_luatex("Umathfixedtopaccent", math_accent_cmd, 5, 0);
+    primitive_luatex("Umathfixedbotaccent", math_accent_cmd, 6, 0);
+    primitive_luatex("Umathfixedaccents", math_accent_cmd, 7, 0);
     primitive_tex("mathchar", math_char_num_cmd, 0, 0);
     primitive_omega("omathchar", math_char_num_cmd, 1, 0);
     primitive_luatex("Umathchar", math_char_num_cmd, 2, 0);
Index: source/texk/web2c/luatexdir/tex/texmath.w
===================================================================
--- source/texk/web2c/luatexdir/tex/texmath.w	(revision 3950)
+++ source/texk/web2c/luatexdir/tex/texmath.w	(working copy)
@@ -715,6 +715,8 @@
         }
         break;
     case accent_noad:
+       switch (subtype(p)) {
+       case 0:
         if (accent_chr(p) != null) {
             if (bot_accent_chr(p) != null) {
                 tprint_esc("Umathaccents");
@@ -729,6 +731,23 @@
             print_fam_and_char(bot_accent_chr(p));
         }
         break;
+       case 1:
+        if (accent_chr(p) != null) {
+            if (bot_accent_chr(p) != null) {
+                tprint_esc("Umathfixedaccents");
+                print_fam_and_char(accent_chr(p));
+                print_fam_and_char(bot_accent_chr(p));
+            } else {
+                tprint_esc("Umathfixedtopaccent");
+                print_fam_and_char(accent_chr(p));
+            }
+        } else {
+            tprint_esc("Umathfixedbotaccent");
+            print_fam_and_char(bot_accent_chr(p));
+        }
+        break;
+       }
+       break;
     }
     print_subsidiary_data(nucleus(p), '.');
     print_subsidiary_data(supscr(p), '^');
@@ -1562,6 +1581,16 @@
     } else if (cur_chr == 4) {  /* \.{\\Umathaccents} */
         t = scan_mathchar(xetex_mathcode);
         b = scan_mathchar(xetex_mathcode);
+    } else if (cur_chr == 5) {  // \Umathfixedtopaccent
+      t = scan_mathchar(xetex_mathcode);
+      subtype(tail) = 1;
+    } else if (cur_chr == 6) {  // \Umathfixedbotaccent
+      b = scan_mathchar(xetex_mathcode);
+      subtype(tail) = 1;
+    } else if (cur_chr == 7) {  // \Umathfixedaccents
+      t = scan_mathchar(xetex_mathcode);
+      b = scan_mathchar(xetex_mathcode);
+      subtype(tail) = 1;
     } else {
         confusion("math_ac");
     }
Index: source/texk/web2c/luatexdir/tex/mlist.w
===================================================================
--- source/texk/web2c/luatexdir/tex/mlist.w	(revision 3950)
+++ source/texk/web2c/luatexdir/tex/mlist.w	(working copy)
@@ -2126,9 +2126,11 @@
 @c
 #define TOP_CODE 1
 #define BOT_CODE 2
+#define TOP_OR_BOT_MASK ((TOP_CODE) | (BOT_CODE))
+#define STRETCH_ACCENT_CODE 4
 
 static void do_make_math_accent(pointer q, internal_font_number f, int c,
-                                int top_or_bot, int cur_style)
+                                int flags, int cur_style)
 {
     pointer p, r, x, y;         /* temporary registers for box construction */
     scaled s;                   /* amount to skew the accent to the right */
@@ -2138,6 +2140,7 @@
     boolean s_is_absolute;      /* will be true if a top-accent is placed in |s| */
     extinfo *ext;
     pointer attr_p;
+    const int top_or_bot = flags & TOP_OR_BOT_MASK;
     attr_p = (top_or_bot == TOP_CODE ? accent_chr(q) : bot_accent_chr(q));
     s_is_absolute = false;
     c = cur_c;
@@ -2168,7 +2171,8 @@
     h = height(x);
     /* Switch to a larger accent if available and appropriate */
     y = null;
-    while (1) {
+    if (flags & STRETCH_ACCENT_CODE) {
+      while (1) {
         ext = NULL;
         if ((char_tag(f, c) == ext_tag) &&
             ((ext = get_charinfo_hor_variants(char_info(f, c))) != NULL)) {
@@ -2187,6 +2191,7 @@
                 break;
             c = yy;
         }
+      }
     }
     if (y == null) {
         y = char_box(f, c, node_attr(attr_p));
@@ -2265,12 +2270,12 @@
     type(nucleus(q)) = sub_box_node;
 }
 
-static void make_math_accent(pointer q, int cur_style)
+static void make_math_accent(pointer q, int cur_style, int stretch)
 {
     if (accent_chr(q) != null) {
         fetch(accent_chr(q));
         if (char_exists(cur_f, cur_c)) {
-            do_make_math_accent(q, cur_f, cur_c, TOP_CODE, cur_style);
+          do_make_math_accent(q, cur_f, cur_c, TOP_CODE | (stretch ? STRETCH_ACCENT_CODE : 0), cur_style);
         }
         flush_node(accent_chr(q));
         accent_chr(q) = null;
@@ -2278,7 +2283,7 @@
     if (bot_accent_chr(q) != null) {
         fetch(bot_accent_chr(q));
         if (char_exists(cur_f, cur_c)) {
-            do_make_math_accent(q, cur_f, cur_c, BOT_CODE, cur_style);
+          do_make_math_accent(q, cur_f, cur_c, BOT_CODE | (stretch ? STRETCH_ACCENT_CODE : 0), cur_style);
         }
         flush_node(bot_accent_chr(q));
         bot_accent_chr(q) = null;
@@ -3417,7 +3422,7 @@
                 make_radical(q, cur_style);
             break;
         case accent_noad:
-            make_math_accent(q, cur_style);
+          make_math_accent(q, cur_style, subtype(q) == 0);
             break;
         case style_node:
             cur_style = subtype(q);