Fix text being incorrectly pre-selected in composer textarea on /share (#17339)

Fixes #17295

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

@@ -60,6 +60,7 @@ class ComposeForm extends ImmutablePureComponent {
     onPickEmoji: PropTypes.func.isRequired,
     showSearch: PropTypes.bool,
     anyMedia: PropTypes.bool,
+    isInReply: PropTypes.bool,
     singleColumn: PropTypes.bool,
   };
 
@@ -149,7 +150,7 @@ class ComposeForm extends ImmutablePureComponent {
     if (this.props.focusDate !== prevProps.focusDate) {
       let selectionEnd, selectionStart;
 
-      if (this.props.preselectDate !== prevProps.preselectDate) {
+      if (this.props.preselectDate !== prevProps.preselectDate && this.props.isInReply) {
         selectionEnd   = this.props.text.length;
         selectionStart = this.props.text.search(/\s/) + 1;
       } else if (typeof this.props.caretPosition === 'number') {

app/javascript/mastodon/features/compose/containers/compose_form_container.js

@@ -25,6 +25,7 @@ const mapStateToProps = state => ({
   isUploading: state.getIn(['compose', 'is_uploading']),
   showSearch: state.getIn(['search', 'submitted']) && !state.getIn(['search', 'hidden']),
   anyMedia: state.getIn(['compose', 'media_attachments']).size > 0,
+  isInReply: state.getIn(['compose', 'in_reply_to']) !== null,
 });
 
 const mapDispatchToProps = (dispatch) => ({