My system: Windows 8.1 + TeX Live 2016 + TXS 2.12.0
Close the structure panel and exit from TXS.
Rerun TXS the structure panel is opened!
Bugs: #2018
Bugs: #2034
Bugs: #2078
Bugs: #2083
Feature Requests: #1223
Same bug on Mac OSX 10.9
fixed: hg 6367 (5e1e2fbe4de7)