Limit compose field height to prevent column scrolling (#39268)

app/javascript/styles/mastodon/components.scss

@@ -614,6 +614,7 @@ body > [data-popper-placement] {
     display: block;
     box-sizing: border-box;
     width: 100%;
+    max-height: 300px;
     margin: 0;
     color: var(--color-text-primary);
     background: transparent;