_content/css: fix content auto-scrolling on selection
_content/css: fix content auto-scrolling on selection

Fix content auto-scrolling when it’s being selected and the cursor is
less than 4.6875rem from the top border of browser view window

Fixes golang/go#52917

Change-Id: Ibf077bbeb3ba57ee425b1965e9e833ba66d11383
Reviewed-on: https://go-review.googlesource.com/c/website/+/409014
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Zeke Lu <04ff0efc18682af42a18b4e27eb24687624bdc4b@gmail.com>
Reviewed-by: Soham Dongargaonkar <dd786caa1486d98255d8c16cb3c9011ee7c5f08d@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website