Fix compose form behavior in mobile view (#15555)

* Fix ComposeForm being mounted twice in mobile view

Fixes #13094

* Fix compose form focus and pre-selection behavior in mobile view

* Split _updateFocusAndSelection out of componentDidUpdate

app/javascript/mastodon/features/compose/components/compose_form.js

@@ -132,7 +132,15 @@ class ComposeForm extends ImmutablePureComponent {
     }
   }
 
+  componentDidMount () {
+    this._updateFocusAndSelection({ });
+  }
+
   componentDidUpdate (prevProps) {
+    this._updateFocusAndSelection(prevProps);
+  }
+
+  _updateFocusAndSelection = (prevProps) => {
     // This statement does several things:
     // - If we're beginning a reply, and,
     //     - Replying to zero or one users, places the cursor at the end of the textbox.