paintedTextHeight method
Height of a cell's laid-out text, or null if it is outside the window.
Observable for the same reason paintedCellCount is: a cell taller than its row paints over its neighbours, and that is a regression a test should be able to catch without reading pixels.
Implementation
double? paintedTextHeight(int row, int column) =>
_byCell[_cellKey(row, column)]?.painter.height;