Entering more than 10 lines in a pinside pm on my phone results in the "send mail" button disappearing. If I delete back to just 10 lines the button reappears.
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Your browser/os information:
Mozilla/5.0 (Linux; Android 7.0; SM-G930T Build/NRD90M) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.116 Mobile Safari/537.36 (this will help us make sense of your bug report)