Add option to apply surround across multilines.

app/assets/javascripts/discourse/components/d-editor.js.es6

@@ -441,7 +441,7 @@ export default Ember.Component.extend({
     }).join("\n");
   },
 
-  _applySurround(sel, head, tail, exampleKey) {
+  _applySurround(sel, head, tail, exampleKey, opts) {
     const pre = sel.pre;
     const post = sel.post;
 
@@ -453,6 +453,16 @@ export default Ember.Component.extend({
       const example = I18n.t(`composer.${exampleKey}`);
       this.set('value', `${pre}${hval}${example}${tail}${post}`);
       this._selectText(pre.length + hlen, example.length);
+    } else if (opts && !opts.multiline) {
+      const [hval, hlen] = getHead(head);
+
+      if (pre.slice(-hlen) === hval && post.slice(0, tail.length) === tail) {
+        this.set('value', `${pre.slice(0, -hlen)}${sel.value}${post.slice(tail.length)}`);
+        this._selectText(sel.start - hlen, sel.value.length);
+      } else {
+        this.set('value', `${pre}${hval}${sel.value}${tail}${post}`);
+        this._selectText(sel.start + hlen, sel.value.length);
+      }
     } else {
       const lines = sel.value.split("\n");
 
@@ -521,7 +531,7 @@ export default Ember.Component.extend({
       const toolbarEvent = {
         selected,
         selectText: (from, length) => this._selectText(from, length),
-        applySurround: (head, tail, exampleKey) => this._applySurround(selected, head, tail, exampleKey),
+        applySurround: (head, tail, exampleKey, opts) => this._applySurround(selected, head, tail, exampleKey, opts),
         applyList: (head, exampleKey) => this._applyList(selected, head, exampleKey),
         addText: text => this._addText(selected, text),
         getText: () => this.get('value'),

plugins/discourse-details/assets/javascripts/initializers/apply-details.js.es6

@@ -18,8 +18,9 @@ function initializeDetails(api) {
         this.get("toolbarEvent").applySurround(
           `[details=${I18n.t("composer.details_title")}]`,
           "[/details]",
-          "details_text")
+          "details_text",
-        ;
+          { multiline: false }
+        );
       }
     }
   });