var custom_mapping = gantt.ext.inlineEditors.getMapping;
Then you replace the default mapping:
case c.LEFT:
case c.RIGHT:
"date" === l.editorType && (d = !0);
t.isVisible() && gantt.getTask(t.getState().id).type == 'placeholder' && (t.setValue());
break;
and
if (t.isVisible()) {
if (r.shiftKey) t.editPrevCell(!0)
else if (gantt.getTask(t.getState().id).type == 'placeholder') (t.setValue(), t.editNextCell(!0))
else t.editNextCell(!0)