See the following images that illustrate the problem:
This image shows an arbitrary message selected by clicking in the blank thread pane to select a message: https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/1504241/+attachment/4488881/+files/thunderbird-thread-pane-bug.png
This image shows the same message by clicking on it in the global inbox thread pane: https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/1504241/+attachment/4488906/+files/thunderbird-thread-pane-bug-2.png
See the following images that illustrate the problem:
This image shows an arbitrary message selected by clicking in the blank thread pane to select a message: /bugs.launchpad .net/ubuntu/ +source/ thunderbird/ +bug/1504241/ +attachment/ 4488881/ +files/ thunderbird- thread- pane-bug. png
https:/
This image shows the same message by clicking on it in the global inbox thread pane: /bugs.launchpad .net/ubuntu/ +source/ thunderbird/ +bug/1504241/ +attachment/ 4488906/ +files/ thunderbird- thread- pane-bug- 2.png
https:/