Export visibleLines to public api

[aslushnikov]

Problem

The only way to figure out a first visible line number (or a bottom visible line number)
is to call codeMirror.coordsChar({left: 0, top: 0}, "local"). (please correct me if I'm wrong).
The problem is, however, that this method is pretty expensive because of a measureLineInner call, so in case of calling it multiple times in a scroll event handler on a file with multiple long lines - scrolling gets laggy.

The visibleLines method seems to be a natural solution for the problem.

[marijnh]

Please see my attached patch instead -- I guess a general way to cheaply find the line at a given y offset would be more generally useful, and less 'random exposed internals'-y.

[aslushnikov]

Does the editor.lineAtHeight(0, "local") return the top visible line? It always returns 0 for me

[marijnh]

No, it returns the first line -- i.e. the line at offset 0. "local" coordinates are not scroll-sensitive.

editor.lineAtHeight(editor.getScrollInfo().top, "local") will probably work better.

[aslushnikov]

Thanks, that works.