Ensure that relative paths in edit preview work (#11143)

Fix #10131

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: Lauris BH <lauris@nix.lv>

web_src/js/index.js

@@ -64,10 +64,16 @@ function initEditPreviewTab($form) {
     previewFileModes = $previewTab.data('preview-file-modes').split(',');
     $previewTab.on('click', function () {
       const $this = $(this);
+      let context = `{$this.data('context')}/`;
+      const treePathEl = $form.find('input#tree_path');
+      if (treePathEl.length > 0) {
+        context += treePathEl.val();
+      }
+      context = context.substring(0, context.lastIndexOf('/'));
       $.post($this.data('url'), {
         _csrf: csrf,
         mode: 'gfm',
-        context: $this.data('context'),
+        context,
         text: $form.find(`.tab.segment[data-tab="${$tabMenu.data('write')}"] textarea`).val()
       }, (data) => {
         const $previewPanel = $form.find(`.tab.segment[data-tab="${$tabMenu.data('preview')}"]`);