[ot-shape] Fix application of kern table if GPOS kern is missing

Was broken sometime after we implemented kerx and tweaking it.

Fixes https://github.com/harfbuzz/harfbuzz/issues/3043

src/hb-ot-shape.cc

@@ -151,7 +151,7 @@ hb_ot_shape_planner_t::compile (hb_ot_shape_plan_t           &plan,
 
   bool has_gsub = hb_ot_layout_has_substitution (face);
   bool has_gpos = !disable_gpos && hb_ot_layout_has_positioning (face);
-  if (0)
+  if (false)
     ;
 #ifndef HB_NO_AAT_SHAPE
   else if (hb_aat_layout_has_positioning (face) && !(has_gsub && has_gpos))
@@ -174,6 +174,8 @@ hb_ot_shape_planner_t::compile (hb_ot_shape_plan_t           &plan,
 #endif
   }
 
+  plan.apply_fallback_kern = !(plan.apply_gpos || plan.apply_kerx || plan.apply_kern);
+
   plan.zero_marks = script_zero_marks &&
 		    !plan.apply_kerx &&
 		    (!plan.apply_kern
@@ -274,11 +276,12 @@ hb_ot_shape_plan_t::position (hb_font_t   *font,
   else if (this->apply_kerx)
     hb_aat_layout_position (this, font, buffer);
 #endif
+
 #ifndef HB_NO_OT_KERN
-  else if (this->apply_kern)
+  if (this->apply_kern)
     hb_ot_layout_kern (this, font, buffer);
 #endif
-  else
+  else if (this->apply_fallback_kern)
     _hb_ot_shape_fallback_kern (this, font, buffer);
 
 #ifndef HB_NO_AAT_SHAPE

src/hb-ot-shape.hh

@@ -112,6 +112,7 @@ struct hb_ot_shape_plan_t
 #else
   static constexpr bool apply_kern = false;
 #endif
+  bool apply_fallback_kern : 1;
 #ifndef HB_NO_AAT_SHAPE
   bool apply_kerx : 1;
   bool apply_morx : 1;

test/shaping/data/in-house/Makefile.sources

@@ -53,6 +53,7 @@ TESTS = \
 	tests/tibetan-contractions-1.tests \
 	tests/tibetan-contractions-2.tests \
 	tests/tibetan-vowels.tests \
+	tests/tt-kern-gpos.tests \
 	tests/use-indic3.tests \
 	tests/use-marchen.tests \
 	tests/use-syllable.tests \

test/shaping/data/in-house/meson.build

@@ -53,6 +53,7 @@ in_house_tests = [
   'tibetan-contractions-1.tests',
   'tibetan-contractions-2.tests',
   'tibetan-vowels.tests',
+  'tt-kern-gpos.tests',
   'use-indic3.tests',
   'use-marchen.tests',
   'use-syllable.tests',

test/shaping/data/in-house/tests/tt-kern-gpos.tests

@@ -0,0 +1 @@
+../fonts/b121d4306b2e3add5abbaad21d95fcf04aacbd64.ttf::U+0041,U+0043,U+0041,U+0042:[A=0+1275|C=1@-20,0+1272|A=2+1296|B=3+1327]