Fix composer header hidden by mobile browser (#2279)

js/src/forum/components/Composer.js

@@ -265,7 +265,7 @@ export default class Composer extends Component {
     this.animateHeightChange().then(() => this.focus());
 
     if (app.screen() === 'phone') {
-      this.$().css('top', $(window).scrollTop());
+      this.$().css('top', 0);
       this.showBackdrop();
     }
   }

less/forum/Composer.less

@@ -114,7 +114,7 @@
     background: @body-bg;
 
     &:not(.minimized) {
-      position: absolute;
+      position: fixed;
       top: 0;
       height: 350px !important;
       padding-top: @header-height-phone;