I have a first working master checkbox but the code is ugly and it does not account for the indeterminate state if child checkboxes are changed.
I found this fomentic example
Would it be cleaner to use a separated javascript file?

needs a Behat test

master checkbox must reflect the child states - unchecked/"partially checked"/"all checked"

if Fomantic-UI/jQuery cannot handle this in a few lines, the JS should be put into js/, but until you have fully working solution, feel free to keep the code uncompiled

Ok, it works as of now with the indeterminate state if the child checkboxes are not all in the same state.

I had some issues to have the javascript statements to appear at the proper position. I first wrote them in Grid->addSelection() and they were called kind of before the checkbox() creation for fomantic-ui.

Still I feel the manner I use the javascript is not optimal because it does not look like other atk4/ui sources.

I did not find any figures of the checkbox column inside the Column/Checkbox or the Grid documentation.

There seem to be no documentation updates to do because the core functionalities of Grid and Column/Checkbox are not affected.

@mhuser, thank for this great contribution and sorry for longer review! ❤

@mvorisek
You are welcome. This is great! 😃