User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:68.0) Gecko/20100101 Firefox/68.0

Steps to reproduce:

Updates to calendar events are causing an error on macOS (10.14.5 (18F132)) when "Update" button is clicked in Thunderbird 67.0b3 (64-bit)

Actual results:

Clicking button to update existing event causes popup window to appear, attached to bottom of program window, with "OK" and "Cancel" options shown. Clicking either button does not dismiss popup. Program remains unresponsive as it is waiting for response from user for the popup. Quitting program does not work, must force-quit to recover from the issue. Screenshot shows issue - when application is full-screen the popup buttons extend past bottom edge of monitor. All calendars/mail accounts involved are Gmail IMAP, if this makes a difference.

Lightning is the only add-on/extension installed in Thunderbird.

Expected results:

Calendar update should simply occur in background as it did in prior versions of Lightning/Thunderbird. Any user action (e.g. accept that calendar event must be reloaded/reprocessed) should occur in popup that is not tied to bottom of program window AND should actually trigger an action when button is pressed.