_content/css: fix editor's scroll on playground

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

Change-Id: I1d4c196d9b3170d766ffdd0df779c5e693a6ed5a
GitHub-Last-Rev: 7727863
GitHub-Pull-Request: #323
Reviewed-on: https://go-review.googlesource.com/c/website/+/679035
Reviewed-by: Sean Liao <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Sean Liao <[email protected]>

Author: ShoshinNikita

_content/css/styles.css

@@ -4581,6 +4581,8 @@ a.error {
   border-top-left-radius: 0.3125rem;
   border-top-right-radius: 0.3125rem;
   overflow: hidden;
+  display: flex;
+  flex-direction: column;
 }
 .PlayPage {
   color: var(--color-text);
@@ -4611,6 +4613,7 @@ a.error {
 }
 .Playground-inputContainer #wrap {
   height: 100%;
+  min-height: 0; /* prevent #wrap from overflowing the container */
 }
 .Playground-outputContainer {
   border-bottom-right-radius: 0.3125rem;