Created attachment 574257 [details] Video showing bug Reproducibility: Always Steps to reproduce (see attached video): 1. Open the message composer window. 2. Enter several recipients (several more than fit vertically) 3. Position the mouse pointer somewhere on the text of the last recipient so that the mouse pointer changes to the I-bar 4. Press the mouse button and drag up Observed behaviour: 5. The text to the left of the I-bar pointer becomes selected. 6. The recipient list scrolls up past the first entry so that the entire section of the window where the recipient list was becomes completely blank. Expected behaviour: 5. The text to the left of the I-bar pointer becomes selected. The recipient list should not permit scroll-dragging. (Or if it does, it shouldn't scroll past the first entry in the list.)
(In reply to Tristan Miller from comment #0) AFAIK, this recipient form is specific to the OS GUI library, so maybe this doesn't happen in Windows or Apple (judging from the Linux-based GUI in your video attachment).
Problem still exists with Thunderbird 52.1.1 on GNU/Linux.