Normally when editing text, cmd-left and cmd-right work like Home and End keys (important on a Macbook which lacks these keys). In the Quick Reply mode, this is overridden by shortcuts for hiding and displaying message detail. So, intending to move the cursor to the start of the line instead hides the message and quick reply entirely! This message detail show/hide shortcut should be disabled when editing in the quick reply box, so the standard home/end shortcuts work properly.