Green Triangle Intersection of Row and Column Headings Under Name Box

[isadoko]

First time see a green triangle at the intersection of the row and column headings under the name box (top left of spreadsheet) suddenly appeared and hid 30 rows. What is it and how to undo it? By the way the triangle am describing is not the cell indicator error. Thanks-

 

[cerfani]

that arrow just selects all cells... not sure how you hid the rows but click that triangle and then in the row headings right click, then click 'unhide'

most actions can be undone by pressing ctrl+z not sure if you knew that but it is a common way to undo in most programs