autocomplete-combobox: fix getting thumbnail size
autocomplete-combobox: fix getting thumbnail size

This got broken at some point on at least chrome.
While before we could query a detached element for
its CSS properties now it has to be attached to the
document in order to do so.

Signed-off-by: Imre Deak <fbd5edba1988036c8923f0cca0cce6ac4811db29@gmail.com>

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