avoid overwriting existing usercss on saving a new one

_locales/en/messages.json

@@ -1259,6 +1259,10 @@
     "message": "Specify @name in the code",
     "description": "Placeholder text for the empty name input field when creating a new Usercss style"
   },
+  "usercssAvoidOverwriting": {
+    "message": "Please change the value of @name or @namespace to avoid overwriting an existing style.",
+    "description": "Shown in a message box when attempting to save a new Usercss style that would overwrite an existing one."
+  },
   "usercssReplaceTemplateConfirmation": {
     "message": "Replace the default template for new Usercss styles with the current code?"
   },

background/usercss-helper.js

@@ -65,10 +65,21 @@
     return style;
   }
 
-  // Parse the source and find the duplication
-  function build({sourceCode, checkDup = false}) {
-    return buildMeta({sourceCode})
-      .then(usercss.buildCode)
+  /**
+   * Parse the source and find the duplication
+   * @param _
+   * @param {String} _.sourceCode
+   * @param {Boolean=} _.checkDup
+   * @param {Boolean=} _.metaOnly
+   * @returns {Promise<{style, dup:Boolean?}>}
+   */
+  function build({
+    sourceCode,
+    checkDup,
+    metaOnly,
+  }) {
+    const task = buildMeta({sourceCode});
+    return (metaOnly ? task : task.then(usercss.buildCode))
       .then(style => ({
         style,
         dup: checkDup && find(style),

edit/source-editor.js

@@ -122,11 +122,11 @@ function createSourceEditor(style) {
     style.sourceCode = '';
 
     chromeSync.getLZValue('usercssTemplate').then(code => {
+      const name = style.name || t('usercssReplaceTemplateName');
+      const date = new Date().toLocaleString();
       code = code || DEFAULT_CODE;
       code = code.replace(/@name(\s*)(?=[\r\n])/, (str, space) =>
-        `${str}${space ? '' : ' '}${
-          style.name ||
-          t('usercssReplaceTemplateName') + ' - ' + new Date().toLocaleString()}`);
+        `${str}${space ? '' : ' '}${name} - ${date}`);
       // strip the last dummy section if any, add an empty line followed by the section
       style.sourceCode = code.replace(/\s*@-moz-document[^{]*\{[^}]*\}\s*$|\s+$/g, '') + '\n\n' + section;
       cm.startOperation();
@@ -202,8 +202,8 @@ function createSourceEditor(style) {
   function save() {
     if (!dirty.isDirty()) return;
     const code = cm.getValue();
-    return (
-      API.saveUsercssUnsafe({
+    return ensureUniqueStyle(code)
+      .then(() => API.saveUsercssUnsafe({
         id: style.id,
         reason: 'editSave',
         enabled: style.enabled,
@@ -214,6 +214,7 @@ function createSourceEditor(style) {
         if (errors) return Promise.reject(errors);
       })
       .catch(err => {
+        if (err.handled) return;
         if (err.message === t('styleMissingMeta', 'name')) {
           messageBox.confirm(t('usercssReplaceTemplateConfirmation')).then(ok => ok &&
             chromeSync.setLZValue('usercssTemplate', code)
@@ -233,6 +234,20 @@ function createSourceEditor(style) {
       });
   }
 
+  function ensureUniqueStyle(code) {
+    return style.id ? Promise.resolve() :
+      API.buildUsercss({
+        sourceCode: code,
+        checkDup: true,
+        metaOnly: true,
+      }).then(({dup}) => {
+        if (dup) {
+          messageBox.alert(t('usercssAvoidOverwriting'), 'danger', t('genericError'));
+          return Promise.reject({handled: true});
+        }
+      });
+  }
+
   function drawLinePointer(pos) {
     const SIZE = 60;
     const line = cm.getLine(pos.line);

msgbox/msgbox.js

@@ -150,10 +150,12 @@ function messageBox({
 /**
  * @param {String|Node|Array<String|Node>} contents
  * @param {String} [className] like 'pre' for monospace font
+ * @param {String} [title]
  * @returns {Promise<Boolean>} same as messageBox
  */
-messageBox.alert = (contents, className) =>
+messageBox.alert = (contents, className, title) =>
   messageBox({
+    title,
     contents,
     className: `center ${className || ''}`,
     buttons: [t('confirmClose')]