Show quote button when post text is selected

closes #19 closes flarum/core#320

extensions/mentions/js/forum/src/components/PostQuoteButton.js

@@ -0,0 +1,40 @@
+import Button from 'flarum/components/Button';
+import extract from 'flarum/utils/extract';
+
+import reply from 'flarum/mentions/utils/reply';
+
+export default class PostQuoteButton extends Button {
+  view() {
+    const post = extract(this.props, 'post');
+    const content = extract(this.props, 'content');
+
+    this.props.className = 'Button PostQuoteButton';
+    this.props.icon = 'quote-left';
+    this.props.children = app.translator.trans('flarum-mentions.forum.post.quote_button');
+    this.props.onclick = () => {
+      this.hide();
+      reply(post, content);
+    };
+    this.props.onmousedown = (e) => e.stopPropagation();
+
+    return super.view();
+  }
+
+  config(isInitialized) {
+    if (isInitialized) return;
+
+    $(document).on('mousedown', this.hide.bind(this));
+  }
+
+  show(left, top) {
+    const $this = this.$();
+
+    $this.show()
+      .css('top', top - $this.outerHeight() - 5)
+      .css('left', left);
+  }
+
+  hide() {
+    this.$().hide();
+  }
+}