Fix rendering of ligatures in the latest release of Cascadia code

For some reason it puts empty glyphs after the ligature glyph rather than before it.
There is a possibility this fix might break something else, we will see.
Fixes #3313

docs/changelog.rst

@@ -64,6 +64,10 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 
 - Fix OS window sizes under 100px resulting in scaled display (:iss:`3307`)
 
+- Fix rendering of ligatures in the latest release of Cascadia code, which for
+  some reason puts empty glyphs after the ligature glyph rather than before it
+  (:iss:`3313`)
+
 
 0.19.3 [2020-12-19]
 -------------------

kitty/fonts.c

@@ -767,7 +767,7 @@ typedef struct {
 
 typedef struct {
     unsigned int first_glyph_idx, first_cell_idx, num_glyphs, num_cells;
-    bool has_special_glyph, is_space_ligature;
+    bool has_special_glyph, is_space_ligature, started_with_empty_glyph;
 } Group;
 
 typedef struct {
@@ -905,6 +905,7 @@ shape_run(CPUCell *first_cpu_cell, GPUCell *first_gpu_cell, index_type num_cells
      * Ligature fonts, take two common approaches:
      * 1. ABC becomes EMPTY, EMPTY, WIDE GLYPH this means we have to render N glyphs in N cells (example Fira Code)
      * 2. ABC becomes WIDE GLYPH this means we have to render one glyph in N cells (example Operator Mono Lig)
+     * 3. ABC becomes WIDE GLYPH, EMPTY, EMPTY this means we have to render N glyphs in N cells (example Cascadia Code)
      *
      * We rely on the cluster numbers from harfbuzz to tell us how many unicode codepoints a glyph corresponds to.
      * Then we check if the glyph is a ligature glyph (is_special_glyph) and if it is an empty glyph. These three
@@ -933,7 +934,8 @@ shape_run(CPUCell *first_cpu_cell, GPUCell *first_gpu_cell, index_type num_cells
             add_to_current_group = true;
         } else {
             if (is_special) {
-                add_to_current_group = G(prev_was_empty);
+                if (current_group->started_with_empty_glyph) add_to_current_group = G(prev_was_empty);
+                else add_to_current_group = is_empty;
             } else {
                 add_to_current_group = !G(prev_was_special);
             }
@@ -941,6 +943,7 @@ shape_run(CPUCell *first_cpu_cell, GPUCell *first_gpu_cell, index_type num_cells
         if (current_group->num_glyphs >= MAX_GLYPHS_IN_GROUP || current_group->num_cells >= MAX_GLYPHS_IN_GROUP) add_to_current_group = false;
 
         if (!add_to_current_group) { G(group_idx)++; current_group = G(groups) + G(group_idx); }
+        if (is_empty && !current_group->num_glyphs) current_group->started_with_empty_glyph = true;
         if (!current_group->num_glyphs++) {
             current_group->first_glyph_idx = G(glyph_idx);
             current_group->first_cell_idx = G(cell_idx);

kitty_tests/fonts.py

@@ -4,6 +4,7 @@
 
 import sys
 import unittest
+from functools import partial
 
 from kitty.constants import is_macos
 from kitty.fast_data_types import (
@@ -71,19 +72,29 @@ class Rendering(BaseTest):
 
     def test_shaping(self):
 
-        def groups(text, path=None):
-            return [x[:2] for x in shape_string(text, path=path)]
+        def ss(text, font=None):
+            path = f'kitty_tests/{font}' if font else None
+            return shape_string(text, path=path)
+
+        def groups(text, font=None):
+            return [x[:2] for x in ss(text, font)]
 
         self.ae(groups('abcd'), [(1, 1) for i in range(4)])
-        self.ae(groups('A=>>B!=C', path='kitty_tests/FiraCode-Medium.otf'), [(1, 1), (3, 3), (1, 1), (2, 2), (1, 1)])
-        self.ae(groups('F--a--', path='kitty_tests/FiraCode-Medium.otf'), [(1, 1), (2, 2), (1, 1), (2, 2)])
-        self.ae(groups('==!=<>==<><><>', path='kitty_tests/FiraCode-Medium.otf'), [(2, 2), (2, 2), (2, 2), (2, 2), (2, 2), (2, 2), (2, 2)])
-        colon_glyph = shape_string('9:30', path='kitty_tests/FiraCode-Medium.otf')[1][2]
-        self.assertNotEqual(colon_glyph, shape_string(':', path='kitty_tests/FiraCode-Medium.otf')[0][2])
+        self.ae(groups('A=>>B!=C', font='FiraCode-Medium.otf'), [(1, 1), (3, 3), (1, 1), (2, 2), (1, 1)])
+        self.ae(groups('==!=<>==<><><>', font='FiraCode-Medium.otf'), [(2, 2), (2, 2), (2, 2), (2, 2), (2, 2), (2, 2), (2, 2)])
+
+        for font in ('FiraCode-Medium.otf', 'CascadiaCode-Regular.otf'):
+            g = partial(groups, font=font)
+            self.ae(g('A===B!=C'), [(1, 1), (3, 3), (1, 1), (2, 2), (1, 1)])
+            self.ae(g('F--a--'), [(1, 1), (2, 2), (1, 1), (2, 2)])
+            self.ae(g('===--<>=='), [(3, 3), (2, 2), (2, 2), (2, 2)])
+        colon_glyph = ss('9:30', font='FiraCode-Medium.otf')[1][2]
+        self.assertNotEqual(colon_glyph, ss(':', font='FiraCode-Medium.otf')[0][2])
         self.ae(colon_glyph, 998)
-        self.ae(groups('9:30', path='kitty_tests/FiraCode-Medium.otf'), [(1, 1), (1, 1), (1, 1), (1, 1)])
+        self.ae(groups('9:30', font='FiraCode-Medium.otf'), [(1, 1), (1, 1), (1, 1), (1, 1)])
+
         self.ae(groups('|\U0001F601|\U0001F64f|\U0001F63a|'), [(1, 1), (2, 1), (1, 1), (2, 1), (1, 1), (2, 1), (1, 1)])
-        self.ae(groups('He\u0347\u0305llo\u0337,', path='kitty_tests/LiberationMono-Regular.ttf'),
+        self.ae(groups('He\u0347\u0305llo\u0337,', font='LiberationMono-Regular.ttf'),
                 [(1, 1), (1, 3), (1, 1), (1, 1), (1, 2), (1, 1)])
 
     def test_emoji_presentation(self):