use hyrarchie in the grid. it works perfect.
question: is it possible to open the main window with all the possible levels already opened ? or is it possible by clicking on a button
and the the same will happen ?
main window shows now:
>level1
>level2
open without clicking like this:
>level1
child1
child2
>level2
chlid3
chlid4

Add a grid column like this example in the documentation. I bet that will work.
http://doc.koolphp.net/Controls/KoolGrid/Javascript/Builtin_Functions/index.php#grid_expand

This answer only for 1 (one) row ! And I do not want on EVERY row a button, one button on the page (not in the grid) must do the work.
Number of total rows in my grid is between 90 and 110 rows, not a nice sight looking at 100 buttons.
But I asked a solution for ALL (master) ROWS in a grid at the same time ! see my example.
Or is it possible to change the text "this" (meaning 1 row) in something so that it will work for the complete grid ?

You said you wanted a button. If you want one button for many rows good luck. If I wanted to expand a row without a button, I would use the row click event. That will eliminate the need for a button. Otherwise, I'm not sure how you will get one row out of many to expand with one button without selecting a row first, and then clicking the button. If you do that, turn on row selecting, then use the button to expand that row. I'm just a user of the product like you. Try something creative with that idea. If it works, let me know.
Good Luck.

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