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The previous tweet was going to end with an emoji, but inserting the emoji took me from 1 char remaining to -3.
Did twitter switch from characters to bytes and switch to the gauge so that it would be less obvious that they switched? 
I just hit insert once and two appeared.
I ran into one UI issue that's arguably not a bug and three bugs while typing these tweets. How many person-years would you guess went into this text box? In the linked thread, the guy said he worked on this for years and it doesn't sound like he was the only one.
We had so many issues like this at Medium, too.
I wouldn't be so quick to blame Twitter. Many bugs wrt spacing and c&p ended up being browser bugs when we dug in (cf https://bugs.chromium.org/p/chromium/issues/detail?id=318925 …)
Not sure if it’s my twitter client or what but I can’t see the tweets you’re referencing by the Twitter developer.
A bug I'm recently running into is that I often can't expand tweets on the website. Reloading often helps, for whatever reason.
Emoji have some awkward encodings. I'm not surprised that it's hard to predict the result. Also, I heard but didn't check that the count is weighted, so less ASCII-like characters use two. The combination might make four.
I used to work on Google Docs. The amount of time spent getting that to behave like Microsoft Word can easily be measured in person-decades. I'm guessing a comparable amount of time has gone into this box.
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