tab "account" renamed to "f", how to fix

[Zhang Weiwu]

hello. Usually in the multi-tabbed view of gnu cash, the first tab is
"accounts", which lists all the account, and is the only tab that
cannot be closed. now I do not know what have I done to make the name of
the tab to "f", and could not manage get back. What i tried are checking
properties (no field value is "f") and right click (where there is no
rename option)