The dictionary returned by `pum_getpos()` uses `scrollbar` not
`scrollable` to indicate whether the scrollbar is visible or not.
In addition, the padding on the left side of the popup menu isn't
included in the output of `pum_getpos()`, meaning that both `col` and
`width` (as understood by nvim-cmp) are off by one when the cursor isn't
at column `0`.
These two issues were causing the documentation window and popup menu to
overlap.
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