function _getLinesInView(textHeight, scrollTop, editorHeight) {
        var scrolledTop    = scrollTop / textHeight,
            scrolledBottom = (scrollTop + editorHeight) / textHeight;

        // Adjust the last line to round inward to show a whole lines.
        var firstLine      = Math.ceil(scrolledTop),
            lastLine       = Math.floor(scrolledBottom) - 1;

        return { first: firstLine, last: lastLine };
    }
@private
Calculates the first and last visible lines of the focused editor
@param {number} textHeight
@param {number} scrollTop
@param {number} editorHeight
@return {{first: number, last: number}}