Selected text in the textarea is selected in VS Code, too, but not quite

### Description and repro

It is nice that selected text in the textarea will be selected in VS Code, too. But it seems to be a few lines off. Also, it would be nice, if the selected portion of the text would be instantly in the viewport in VS Code, if that is possible at all.

### Browser(s) tried

Arc, Chrome

### Editor(s) tried

VS Code

Reproduce the selection-sync behavior from a textarea in Arc or Chrome while using VS Code, checking the reported line offset and viewport position. Trace the browser-to-editor selection handling, then verify that the corresponding text is aligned and brought into the VS Code viewport when possible.

Written by the indexing model from the issue text.