Moves count visible lines forward, if possible.
If count would move past the start or end of the buffer, moves to the start or end of the buffer.
The return value indicates whether the iterator moved onto a dereferenceable position; if the iterator didn’t move, or moved onto the
end iterator, then false is returned. If count is 0, the function does nothing and returns
false. If count is negative, moves backward by 0 - count lines.
| this |
a `GtkTextIter` |
| count |
number of lines to move forward |
|
whether this moved and is dereferenceable |