Review all the uses of `_initialY` and verify if the uses are safe when `_initialY` is negative

[daxian-dbw]

Given _initialY could be negative (e.g. multiple-line text editing causes the initial cursor position to be scrolled off the buffer), we need to review all the uses of _initialY and verify if the uses are safe when _initialY is negative.

[daxian-dbw]

I reviewed all references to _initialY, and nothing obvious stands out. Also I reviewed all new issues and it looks none of them is related to _initialY becoming negative.
I tended to believe there is nothing we should do for now. If something related to this comes up, we will fix it at the time.