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1. Write a 280 char tweet w/spaces
2. ctrl+a, ctrl+x
3. ctrl+v
4. Tweet becomes 283 to 284 chars
This is consistently reproducible for me in FF.
In the linked thread, a guy who worked on this feature explains why it's good that undo, cut, copy, etc. don't work correctly 
https://twitter.com/garybernhardt/status/891487984465268737 …
If you've ever wondered why I sometimes have tweets with extra spaces in them, I habitually copy as I type as a "backup". When the web app blows up, I paste to "restore" but twitter breaks this. I remove the extra spaces I can spot, but it's hard to spot if it's at a line break
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.
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