Add test for Tai Tham letter position.y difference

src/font/shaper/coretext.zig

@@ -1533,6 +1533,66 @@ test "shape Tai Tham vowels (position differs from advance)" {
     try testing.expectEqual(@as(usize, 1), count);
 }
 
+test "shape Tai Tham letters (position.y differs from advance)" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    // We need a font that supports Tai Tham for this to work, if we can't find
+    // Noto Sans Tai Tham, which is a system font on macOS, we just skip the
+    // test.
+    var testdata = testShaperWithDiscoveredFont(
+        alloc,
+        "Noto Sans Tai Tham",
+    ) catch return error.SkipZigTest;
+    defer testdata.deinit();
+
+    var buf: [32]u8 = undefined;
+    var buf_idx: usize = 0;
+
+    // First grapheme cluster:
+    buf_idx += try std.unicode.utf8Encode(0x1a49, buf[buf_idx..]); // HA
+    buf_idx += try std.unicode.utf8Encode(0x1a60, buf[buf_idx..]); // SAKOT
+    // Second grapheme cluster, combining with the first in a ligature:
+    buf_idx += try std.unicode.utf8Encode(0x1a3f, buf[buf_idx..]); // YA
+    buf_idx += try std.unicode.utf8Encode(0x1a69, buf[buf_idx..]); // U
+
+    // Make a screen with some data
+    var t = try terminal.Terminal.init(alloc, .{ .cols = 30, .rows = 3 });
+    defer t.deinit(alloc);
+
+    // Enable grapheme clustering
+    t.modes.set(.grapheme_cluster, true);
+
+    var s = t.vtStream();
+    defer s.deinit();
+    try s.nextSlice(buf[0..buf_idx]);
+
+    var state: terminal.RenderState = .empty;
+    defer state.deinit(alloc);
+    try state.update(alloc, &t);
+
+    // Get our run iterator
+    var shaper = &testdata.shaper;
+    var it = shaper.runIterator(.{
+        .grid = testdata.grid,
+        .cells = state.row_data.get(0).cells.slice(),
+    });
+    var count: usize = 0;
+    while (try it.next(alloc)) |run| {
+        count += 1;
+
+        const cells = try shaper.shape(run);
+        try testing.expectEqual(@as(usize, 3), cells.len);
+        try testing.expectEqual(@as(u16, 0), cells[0].x);
+        try testing.expectEqual(@as(u16, 0), cells[1].x);
+        try testing.expectEqual(@as(u16, 1), cells[2].x); // U from second grapheme
+
+        // The U glyph renders at a y below zero
+        try testing.expectEqual(@as(i16, -3), cells[2].y_offset);
+    }
+    try testing.expectEqual(@as(usize, 1), count);
+}
+
 test "shape box glyphs" {
     const testing = std.testing;
     const alloc = testing.allocator;