Update placeholder visibility during composition
Update placeholder visibility during composition

Closes #6420:

dperetti/CodeMirror,nightwing/CodeMirror,hackmdio/CodeMirror,hackmdio/CodeMirror,wingify/CodeMirror,nightwing/CodeMirror,pabloferz/CodeMirror,pabloferz/CodeMirror,dperetti/CodeMirror,dperetti/CodeMirror,wingify/CodeMirror,pabloferz/CodeMirror,hackmdio/CodeMirror,nightwing/CodeMirror,wingify/CodeMirror