Add: save usercss from multi-section editor

_locales/en/messages.json

@@ -787,10 +787,18 @@
     "message": "Enter a name.",
     "description": "Error displayed when user saves without providing a name"
   },
+  "stylePasteMozillaFormat": {
+    "message": "The code you pasted looks like mozilla format style. Would you like to import it?",
+    "description": "The confirmation to import mozilla format style from pasted text"
+  },
   "styleSaveLabel": {
     "message": "Save",
     "description": "Label for save button for style editing"
   },
+  "styleSaveAsUsercss": {
+    "message": "The source code contains usercss metadata. Would you like to save the source code as usercss?",
+    "description": "The confirmation to create usercss from the editor"
+  },
   "styleSectionsTitle": {
     "message": "Sections",
     "description": "Title for the style sections section"

edit/edit.js

@@ -306,7 +306,8 @@ function setupCodeMirror(textarea, index) {
     if (
       text.includes('@-moz-document') &&
       text.replace(/\/\*[\s\S]*?\*\//g, '')
-        .match(/@-moz-document[\s\r\n]+(url|url-prefix|domain|regexp)\(/)
+        .match(/@-moz-document[\s\r\n]+(url|url-prefix|domain|regexp)\(/) &&
+      confirm(t('stylePasteMozillaFormat'))
     ) {
       event.preventDefault();
       fromMozillaFormat();
@@ -1476,7 +1477,28 @@ function save() {
   editor.save();
 }
 
+function isUsercssSourceCode() {
+  const sourceCode = editors[0].getValue();
+  return /==UserStyle==[\s\S]+==\/UserStyle==/i.test(sourceCode);
+}
+
 function saveSectionStyle() {
+  if (!styleId && isUsercssSourceCode() && confirm(t('styleSaveAsUsercss'))) {
+    const sourceCode = editors[0].getValue();
+    onBackgroundReady()
+      .then(() => BG.usercssHelper.save({
+        reason: 'editSave',
+        sourceCode,
+        enabled: $('#enabled').checked
+      }))
+      .then(style => {
+        window.onbeforeunload = null;
+        location.href = `/edit.html?id=${style.id}`;
+      })
+      .catch(alert);
+    return;
+  }
+
   updateLintReportIfEnabled(null, 0);
 
   // save the contents of the CodeMirror editors back into the textareas