We are planning to release tomorrow or in the coming week. If everything goes well it'll be available in this release. Otherwise you'll have to wait for the next release. We send a patch release almost every 2 to 3 weeks.
I checked and it works fine when you have not set the field config in the selection mode. Is there a reason that you have used this config field : 'selectionColumn'?
The below code works just fine.
selectionMode : {
checkbox : {
text : 'Choose',
type : 'check',
},
checkboxOnly : true,
showCheckAll : true
},
The checkbox selection column is not a fully normal CheckColumn. It is not supposed to have any field configured, because it is not linked to record data (it is linked to grid selection). We will clarify that in docs.
And to get access to it at runtime, you can use grid.columns.first (if you also have a RowNumberColumn, use grid.columns.getAt(1))
We have lot of implementations that rely on the field config and it use to work in 5.11.1 version but after upgrading we are experiencing the described issue.