Certainly understand and appreciate that! So, it's actually your posts and not the post editor box that's giving you problems. Are you copying and pasting at all in the post? Did you select the right justify icon maybe? If it's limited to that particular thread, it's most likely an issue with the html source code getting "messed up" if members are copying and pasting an original post. If you can let me know if this problem is limited to this thread and not happening when you post elsewhere, that would be super helpful.Â
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Just for future reference, I have a Mac, too, and when I want to do a screenshot, I click command+option+4 when I want to capture a portion of something and click command+option+3 to take a full screen picture. But, your link is perfect as an example so no screen shot necessary 
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Thank you so much for providing me with an example of what's going on for you. If you see others talking about this problem, can you please let them know to post in here so we can try and narrow down what and why it's occurring? Again, I'm really sorry for the inconvenience!Â