Seems that the setting variableRowHeight would do the job? Or is it not the right version/available?
https://docs.sencha.com/extjs/6.5.3/classic/Ext.grid.column.Column.html#cfg-variableRowHeight
https://forum.sencha.com/forum/showthread.php?470059-FIXED-ExtJS-6-5-Scroll-issue-in-grid-with-variable-height-rows