There is a problem with the function VisibleColumnCount, it returns a wrong value because FCStart and FCStop are both set to 0 at the time they are called in the attached test project.
Also in the attached archive you'll find a modified FMX.TMSCustomGrid unit with changes I had to apply after updating to the latest version made available 126.96.36.199. The project wouldn't compile due to unknown identifier error in the fixed grid cell class. Search for 'ctre' to find the commented out code.
VisibleCols.zip (70.6 KB)