_content/css: fix editor's scroll on playground

[ShoshinNikita]

Use "display: flex" to account for the height of the "Press Esc to move..."
banner when calculating the height of the editor.

This fixes the bug that prevented users from scrolling to the end of
the text.

Fixes golang/go#63237

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Nikita Shoshin:

Patch Set 2:

(1 comment)

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Message from Sean Liao:

Patch Set 2: Auto-Submit+1 Code-Review+2 Commit-Queue+1

(1 comment)

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Message from Go LUCI:

Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-06-07T10:12:59Z","revision":"5af6ddeaf0a08e1a7b4be96695d734ca215c95f3"}

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Message from Sean Liao:

Patch Set 2: -Commit-Queue

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Message from Go LUCI:

Patch Set 2:

This CL has passed the run

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Patch Set 2: LUCI-TryBot-Result+1

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Message from Cherry Mui:

Patch Set 2: Code-Review+1

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This PR is being closed because golang.org/cl/679035 has been merged.