The cell tooltip works well when the text is small and it displays above the cell. But when it is a large amount of text it is only ok if you avoid interacting with it. If you move your mouse over it (to scroll the content for example) it disappears and then flashes back into sight (since you are likely still hovering on the cell). So it flashes on and off causing a bit of an issue.
Support Forum
We have this ticket open to track this issue: https://github.com/bryntum/support/issues/2170
You can try setting a maxHeight to something like 70% to avoid the tip growing too tall.
The best way to do this until the fixed version comes out would be to configure the cellTooltip feature like this:
cellTooltip : {
align : {
align : 'b-t',
minHeight : 300
}
}
The align 'b-t' is the current default, it means the tooltip's bottom align's to its target's (the cell's) top.
The minHeight is the addition here. It indicates that it is willing to shrink in height in order to conform to that alignment.
So that means the tooltip will be fitted into available space above the cell, but if that available space is less than 300, it will flip to being below the cell.
You can experiment with aligning and how all the various constraining factors play into it here: https://www.bryntum.com/examples/grid/tooltip/