Inline completer ghost removal causes unnecessary editor refresh when no ghosts are present

[krassowski]

Description

Even after disabling the cell toolbar (#15971), switching cells can take over 35ms. There is 5-6ms which should be recovered by not triggering code mirror logic when no ghost text is present:

Reproduce

See #15465 and #15971 and

Expected behavior

The ghost text logic does not trigger expensive CodeMirror logic when not needed. Ideally it is not added as an extension if all inline completion providers are disabled.

Context

• Browser and version: Chrome 120+
• JupyterLab version: 4.1.4