ᐅ Filling the gap between terrace slabs and window sill

The gap between the terrace slabs and the threshold of the terrace door in my case is about 1-2cm (0.4-0.8 inches). I am concerned that someone stepping accidentally on the threshold might loosen it, so I want to fully support it underneath. The threshold slopes downward toward the front. Installing, for example, a wooden strip is not possible because due to the slope, I can’t fit anything underneath. In my opinion, the only option left is to use construction foam. However, I am worried that as it expands, it might push the threshold upward and potentially loosen it as well.

How would you fill this gap, and with which material? It should provide some resistance between the threshold and the terrace slabs, but it should also be somewhat flexible to accommodate material expansion caused by heat or cold.

Do you have any ideas? Thanks