FIX: Disable overflow-anchor on composer textarea (#23875)

This fixes an issue where, on a textarea with a lot of text, the cursor
would jump when adding a new line. See video in PR for a repro.

This is a Chrome bug with scroll anchoring.

Refs: https://bugs.chromium.org/p/chromium/issues/detail?id=997266

The fix here disables `overflow-anchor` on the composer textarea. There
should be no side effects to this change, as scroll anchoring is likely
not needed for the composer textarea element.

app/assets/stylesheets/common/d-editor.scss

@@ -86,6 +86,7 @@
   height: 100%;
   overflow-x: hidden;
   overscroll-behavior: contain;
+  overflow-anchor: none;
   resize: none;
 }