Taken from the Lodestone...

"The following changes are planned for release in 1.18a. All of these are still in development, and therefore subject to change.



The log will automatically be hidden at the following times:
during event cutscenes
while the main map is open

* The log will remain if a tab is active when a cutscene begins or the main map is opened.

Once hidden, the log will reappear at the following times:
an Open chat prompt command is input (Space bar, slash (/) key, Ctrl+S, etc.)
Configuration » Chat Log appears in the active log tab. (This does not apply when the active tab is the battle tab.)

* The log will be hidden after being inactive for a period of 5 seconds.

Default log settings will be changed to the following:
Two logs will be displayed. The second tab of the first log will be inactive, and the first　tab of the second log will be active.
Log size will be adjusted.
Log transparency will be adjusted.

* The above settings are applied whenever the game is launched, or upon selecting the Restore Defaults option under Configuration » Chat Log.
* The above settings are not applied when saved log configuration data is present."

Source: The Lodestone