help: Add documentation on resizing the compose box.

Co-authored-by: Alya Abbott <alya@zulip.com>

Fixes #20017.

templates/zerver/help/include/sidebar_index.md

@@ -56,6 +56,7 @@
 
 ## Sending messages
 * [Open the compose box](/help/open-the-compose-box)
+* [Resize the compose box](/help/resize-the-compose-box)
 * [Format messages using Markdown](/help/format-your-message-using-markdown)
 * [Mention a user or group](/help/mention-a-user-or-group)
 * [Preview messages before sending](/help/preview-your-message-before-sending)

templates/zerver/help/open-the-compose-box.md

@@ -37,3 +37,4 @@ can close the compose box using `Esc`, or up/down arrow if the box is empty.
 * [Replying to messages](/help/replying-to-messages)
 * [Messaging tips & tricks](/help/messaging-tips)
 * [Keyboard shortcuts](/help/keyboard-shortcuts)
+* [Resize the compose box](/help/resize-the-compose-box)

templates/zerver/help/resize-the-compose-box.md

@@ -0,0 +1,39 @@
+# Resize the compose box
+
+When composing a long message, it can be helpful to expand the compose
+box to display more text and avoid distractions. Note that the compose
+box also stretches automatically when you type a long message.
+
+## Expand the compose box
+
+{start_tabs}
+
+{!start-composing.md!}
+
+1. Click on the **<i class="fa fa-chevron-up"></i> icon** in the top
+   right corner of the compose box.
+
+{end_tabs}
+
+## Collapse the compose box
+
+{start_tabs}
+
+1. With the compose box in the expanded state, click on the **<i class="fa
+   fa-chevron-down"></i> icon** in the top right corner of the compose box.
+
+{end_tabs}
+
+## Stretch the compose box
+
+{start_tabs}
+
+{!start-composing.md!}
+
+1. Click and drag the bottom right corner of the compose box.
+
+{end_tabs}
+
+## Related articles
+
+* [Open the compose box](/help/open-the-compose-box)