We have various tests in blink that test processing of emojis (or other non-BMP unicode chars). Here's one of the tests as processed by clang-format, compared to the same test with each emoji replaced with an ansi char:

thakis@thakis:~/src/chrome/src$ cat test.cc
TEST_F(SymbolsIteratorTest, AllEmojiZWSSequences)
{
    CHECK_RUNS({ { "abcdefghijklmnopqrstuvwxyzabcdefghij"
        "klmnopqrstuvxyzabcdefghijklmnopqrstuvwxyzabcdefghilklmnopqrstuvwx"
        "yzabcdefghijklmnopqrstuvwxyzabcdef",
        FontFallbackPriority::EmojiEmoji } });
}

TEST_F(SymbolsIteratorTest, AllEmojiZWSSequences)
{
    CHECK_RUNS({ { "💏👩‍❤️‍💋‍👨👨‍❤️‍💋‍👨👩‍❤️‍💋‍👩💑👩‍❤️‍👨👨‍❤️‍👨👩‍❤️"
        "‍👩👪👨‍👩‍👦👨‍👩‍👧👨‍👩‍👧‍👦👨‍👩‍👦‍👦👨‍👩‍👧‍👧👨‍👨‍👦👨‍👨‍👧👨‍👨‍👧‍👦👨‍👨‍👦‍👦👨‍👨‍👧‍👧"
        "👩‍👩‍👦👩‍👩‍👧👩‍👩‍👧‍👦👩‍👩‍👦‍👦👩‍👩‍👧‍👧👁‍🗨",
        FontFallbackPriority::EmojiEmoji } });
}
thakis@thakis:~/src/chrome/src$ buildtools/linux64/clang-format test.cc
TEST_F(SymbolsIteratorTest, AllEmojiZWSSequences) {
  CHECK_RUNS(
      {{"abcdefghijklmnopqrstuvwxyzabcdefghij"
        "klmnopqrstuvxyzabcdefghijklmnopqrstuvwxyzabcdefghilklmnopqrstuvwx"
        "yzabcdefghijklmnopqrstuvwxyzabcdef",
        FontFallbackPriority::EmojiEmoji}});
}

TEST_F(SymbolsIteratorTest, AllEmojiZWSSequences) {
  CHECK_RUNS(
      {{"💏👩‍❤️‍💋‍👨👨‍❤️‍💋‍👨👩‍❤️‍💋‍👩💑👩‍❤️‍👨👨‍❤️"
        "‍👨👩‍❤️"
        "‍👩👪👨‍👩‍👦👨‍👩‍👧👨‍👩‍👧‍👦👨‍👩‍👦‍👦👨‍👩‍👧‍👧👨‍👨‍"
        "👦👨‍👨‍👧👨‍👨‍👧‍👦👨‍👨‍👦‍👦👨‍👨‍👧‍"
        "👧"
        "👩‍👩‍👦👩‍👩‍👧👩‍👩‍👧‍👦👩‍👩‍👦‍👦👩‍👩‍👧‍👧👁‍"
        "🗨",
        FontFallbackPriority::EmojiEmoji}});
}


Note all the pointless linebreaks in the emoji version.


What's worse, if I iterate `clang-format -i test.cc`, then clang-format will move one single char from the end of the first string in its own line on every run, so that I'll eventually end up with:


TEST_F(SymbolsIteratorTest, AllEmojiZWSSequences) {
  CHECK_RUNS(
      {{"💏👩‍❤️‍💋‍👨👨‍❤️‍💋‍👨👩‍❤"
        "️"
        "‍"
        "💋"
        "‍"
        "👩"
        "💑"
        "👩"
        "‍"
        "❤"
        "️"
        "‍"
        "👨"
        "👨"
        "‍"
        "❤"
        "️"
        "‍👨👩‍❤️"
        "‍👩👪👨‍👩‍👦👨‍👩‍👧👨‍👩‍👧‍"
        "👦"
        "👨"
        "‍"
        "👩"
        "‍"
        "👦"
        "‍"
        "👦"
        "👨"
        "‍"
        "👩"
        "‍"
        "👧"
        "‍"
        "👧"
        "👨"
        "‍"
        "👨"
        "‍"
        "👦👨‍👨‍👧👨‍👨‍👧‍👦👨‍👨‍👦‍"
        "👦"
        "👨"
        "‍"
        "👨"
        "‍"
        "👧"
        "‍"
        "👧"
        "👩‍👩‍👦👩‍👩‍👧👩‍👩‍👧‍👦👩‍"
        "👩"
        "‍"
        "👦"
        "‍"
        "👦"
        "👩"
        "‍"
        "👩"
        "‍"
        "👧"
        "‍"
        "👧"
        "👁"
        "‍"
        "🗨",
        FontFallbackPriority::EmojiEmoji}});
}

That seems to be the fixpoint, after that form clang-format stops changing the file.