[v4] Experiment with a partial algorithm for redrawing lines

[marijnh]

When a single character is typed into a 5000-character line, the DOM structure for the whole (text part of the) line is reconstructed. This has good reasons (it simplifies a lot of things), but is a major source of the slowness of long lines.

With the new, more descriptive way of keeping track of the visible window in the v4 branch, it should be possible to compute a 'DOM diff' of the actual change that is needed, and patch, rather than redraw, the DOM element.

[adrianheine]

This is related to #1356, right (although that is only about wrapped lines and this probably also applies to un-wrapped editing)? Is this something we could do / already are doing in the rewrite?

[marijnh]

I've opened codemirror/dev#29 to track this in the new architecture