"Multiline" placeholder results in multiline cursor

Created by: fkling

Due to how CodeMirror's default placeholder extension works, having a placeholder that spans multiple lines causes a single tall cursor to be rendered (since the document is actually empty and only has a single line).

For more context: CodeMirror's default placeholder extension doesn't modify the editor content but renders a widget decoration at position 0, which contains the placeholder text. Therefore a "multi-line" placeholder is really just a single-line one that happens to wrap and cause the single line to be taller.

We should file an official bug report (if necessary) but also look into alternative approaches.