When I double click the top left visible emty cell of a sheet, my "LibreOffice 3.4.1RC1 – WIN7 Home Premium (64bit) German UI [OOO340m1 (Build:101)]" shows a black bar horizontally centered with 1/3 cell width and full cell height. Same after <f2>. I did not observe any "real" problem. That behavior has been confirmed on mailing list "[de-users] CALC Merkwuerdiger schwarzer Kasten in Zelle" for WIN XP32, .deb 64 @Kohei: Please feel free to reassign if it’s not your area or if provided information is not sufficient!
Created attachment 48226 [details] Black bar in cell A1
This is a dupe. It only happens in the top-left cell and when it's empty. *** This bug has been marked as a duplicate of bug 38042 ***