The problem is when your dropdown component (D) is in a scrolling container (2) in the page (1).

    -1-----------
    |           |
    |  -2---    |
    |  | D |    | 
    |  -----    |
    |           |
    -------------

If you attach the dropdown options to the inner container (2), they are cropped.

If you attach the dropdown options to the outer container (1), they don't scroll with the rest of their component (D).

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Non-native dropdowns in general are handicapped.

For Windows/MacOS/Linux, native dropdowns can exceed the bounds of even the OS window. There is no CSS equivalent of this behavior.