Currently, the input field goes off-screen if the read-only prefix is too long.
Given that the prefix is read-only, the user probably knows what the prefix is from context. e.g. In the case of the Repost Modal, the prefix is repeated in a selection field above. So, it's probably ok to just truncate the text to an ellipsis.
The input field is the more important one here, so giving it 60% width when truncation is needed.
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