diff --git a/src/hb-buffer-private.hh b/src/hb-buffer-private.hh
index c5a116d84..574fb36c6 100644
--- a/src/hb-buffer-private.hh
+++ b/src/hb-buffer-private.hh
@@ -43,7 +43,8 @@ template <> class hb_mark_as_flags_t<hb_buffer_serialize_flags_t> {};
 
 enum hb_buffer_scratch_flags_t {
   HB_BUFFER_SCRATCH_FLAG_DEFAULT			= 0x00000000u,
-  HB_BUFFER_SCRATCH_FLAG_HAS_SPACE_FALLBACK		= 0x00000001u,
+  HB_BUFFER_SCRATCH_FLAG_HAS_DEFAULT_IGNORABLES		= 0x00000001u,
+  HB_BUFFER_SCRATCH_FLAG_HAS_SPACE_FALLBACK		= 0x00000002u,
 };
 template <> class hb_mark_as_flags_t<hb_buffer_scratch_flags_t> {};
 
diff --git a/src/hb-ot-layout-private.hh b/src/hb-ot-layout-private.hh
index 242d5cc5a..60fca26e2 100644
--- a/src/hb-ot-layout-private.hh
+++ b/src/hb-ot-layout-private.hh
@@ -240,8 +240,9 @@ enum hb_unicode_props_flags_t {
 template <> class hb_mark_as_flags_t<hb_unicode_props_flags_t> {};
 
 static inline void
-_hb_glyph_info_set_unicode_props (hb_glyph_info_t *info, hb_unicode_funcs_t *unicode)
+_hb_glyph_info_set_unicode_props (hb_glyph_info_t *info, hb_buffer_t *buffer)
 {
+  hb_unicode_funcs_t *unicode = buffer->unicode;
   unsigned int u = info->codepoint;
   unsigned int gen_cat = (unsigned int) unicode->general_category (u);
   unsigned int props = gen_cat;
@@ -250,6 +251,7 @@ _hb_glyph_info_set_unicode_props (hb_glyph_info_t *info, hb_unicode_funcs_t *uni
   {
     if (unlikely (unicode->is_default_ignorable (u)))
     {
+      buffer->scratch_flags |= HB_BUFFER_SCRATCH_FLAG_HAS_DEFAULT_IGNORABLES;
       props |=  UPROPS_MASK_IGNORABLE;
       if (u == 0x200Cu) props |= UPROPS_MASK_ZWNJ;
       if (u == 0x200Du) props |= UPROPS_MASK_ZWJ;
diff --git a/src/hb-ot-shape-normalize.cc b/src/hb-ot-shape-normalize.cc
index 116291622..9317b8439 100644
--- a/src/hb-ot-shape-normalize.cc
+++ b/src/hb-ot-shape-normalize.cc
@@ -99,7 +99,7 @@ output_char (hb_buffer_t *buffer, hb_codepoint_t unichar, hb_codepoint_t glyph)
 {
   buffer->cur().glyph_index() = glyph;
   buffer->output_glyph (unichar); /* This is very confusing indeed. */
-  _hb_glyph_info_set_unicode_props (&buffer->prev(), buffer->unicode);
+  _hb_glyph_info_set_unicode_props (&buffer->prev(), buffer);
 }
 
 static inline void
@@ -399,7 +399,7 @@ _hb_ot_shape_normalize (const hb_ot_shape_plan_t *plan,
       /* Modify starter and carry on. */
       buffer->out_info[starter].codepoint = composed;
       buffer->out_info[starter].glyph_index() = glyph;
-      _hb_glyph_info_set_unicode_props (&buffer->out_info[starter], buffer->unicode);
+      _hb_glyph_info_set_unicode_props (&buffer->out_info[starter], buffer);
 
       continue;
     }
diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index 0b1417283..30563d0bd 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -228,7 +228,7 @@ hb_set_unicode_props (hb_buffer_t *buffer)
   unsigned int count = buffer->len;
   hb_glyph_info_t *info = buffer->info;
   for (unsigned int i = 0; i < count; i++)
-    _hb_glyph_info_set_unicode_props (&info[i], buffer->unicode);
+    _hb_glyph_info_set_unicode_props (&info[i], buffer);
 }
 
 static void
@@ -245,7 +245,7 @@ hb_insert_dotted_circle (hb_buffer_t *buffer, hb_font_t *font)
 
   hb_glyph_info_t dottedcircle = {0};
   dottedcircle.codepoint = 0x25CCu;
-  _hb_glyph_info_set_unicode_props (&dottedcircle, buffer->unicode);
+  _hb_glyph_info_set_unicode_props (&dottedcircle, buffer);
 
   buffer->clear_output ();
 
@@ -416,7 +416,8 @@ hb_ot_zero_width_default_ignorables (hb_ot_shape_context_t *c)
 {
   hb_buffer_t *buffer = c->buffer;
 
-  if (buffer->flags & HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES)
+  if (!(buffer->scratch_flags & HB_BUFFER_SCRATCH_FLAG_HAS_DEFAULT_IGNORABLES) ||
+      (buffer->flags & HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES))
     return;
 
   unsigned int count = buffer->len;
@@ -433,7 +434,8 @@ hb_ot_hide_default_ignorables (hb_ot_shape_context_t *c)
 {
   hb_buffer_t *buffer = c->buffer;
 
-  if (buffer->flags & HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES)
+  if (!(buffer->scratch_flags & HB_BUFFER_SCRATCH_FLAG_HAS_DEFAULT_IGNORABLES) ||
+      (buffer->flags & HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES))
     return;
 
   unsigned int count = buffer->len;
diff --git a/src/hb-private.hh b/src/hb-private.hh
index c65a8bf59..5ed7ff3b2 100644
--- a/src/hb-private.hh
+++ b/src/hb-private.hh
@@ -892,6 +892,9 @@ hb_in_ranges (T u, T lo1, T hi1, T lo2, T hi2, T lo3, T hi3)
 
 
 /* Enable bitwise ops on enums marked as flags_t */
+/* To my surprise, looks like the function resolver is happy to silently cast
+ * one enum to another...  So this doesn't provide the type-checking that I
+ * originally had in mind... :( */
 template <class T> class hb_mark_as_flags_t;
 template <class T> static inline T operator | (T l, T r)
 { hb_mark_as_flags_t<T> unused HB_UNUSED; return T ((unsigned int) l | (unsigned int) r); }