I do not know what his code and display is; however when I display, the ONLY thing I do is display.
When new data comes down the pipe it must ask for transfer permission which is not given till the display cycle is completed. All I do is to check a flag set by a (correctly coded i.e. very small) Transfer request detect ISr at the end of the cycle and, if set abandon all display activities till the cause has been processed. This leads to an occasional blackout of about 50mS (20HZ) that nobody notices since the display changes.

Erik