Take lanes auto collapse / expand suddenly ? [solved]

EDIT for the record : It's " Shift+Z ".
Hello all,
 
I don't realy know why but suddenly my take lanes are auto collapsing and expanding... This is driving me totaly crazy ! I don't find any clue on how to disable it.
When i select a track, it auto expand its take lanes and auto collapse other tracks take lanes so i have only one track expanded at a time.
I would like sonar to get back to it's normal behaviour when i can expand / collapse any track i want without affecting the others, but can't find how.
 
Thanks in advance for you help.
J.