Continue the tile underneath it. Your tiler should be able to cut it so it fits.

you can use the same material as countertop just under the window up to the trim. or provided you can make it work with the handle/opener make a window ledge from countertop and run the splash up to it.

a little different and higher than yours and still unfinished but you get the idea.

The tile is fine, the installer can cut tiles and keep the pattern going.