Classic Editor: set background, positioning and height for Skip to Editor link

[sabernhardt]

• Assigns the right positioning value regardless of whether the link has focus, to avoid horizontal scroll.
• Sets the background color to white.
• Reduces the line-height and min-height.
• Keeps the top value of 4 pixels for any screen width.

Trac 64727

Use of AI Tools: none

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[github-actions]

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Props sabernhardt.

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[sabernhardt]

The 32-pixel height matches the plugin action buttons (since r61729).

[sabernhardt]

Trunk, before patch	With patch
With focus, at 640px and 1000px	With focus, at 640px and 1000px
No focus, at 640px	No focus, at 640px

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For comparison, 6.9.1 had a smaller link:

And the horizontal scroll was less noticeable:

[sabernhardt]

Committed in r61742