Placeholder and improve consistency of stylesheet rule priorities

ext/bg/js/settings-popup-preview.js

@@ -21,6 +21,7 @@ class SettingsPopupPreview {
     constructor() {
         this.frontend = null;
         this.apiOptionsGetOld = apiOptionsGet;
+        this.popupInjectOuterStylesheetOld = Popup.injectOuterStylesheet;
         this.popupShown = false;
         this.themeChangeTimeout = null;
     }
@@ -56,6 +57,9 @@ class SettingsPopupPreview {
 
         await this.frontend.isPrepared();
 
+        // Overwrite popup
+        Popup.injectOuterStylesheet = (...args) => this.popupInjectOuterStylesheet(...args);
+
         // Update search
         this.updateSearch();
     }
@@ -76,6 +80,19 @@ class SettingsPopupPreview {
         return options;
     }
 
+    popupInjectOuterStylesheet(...args) {
+        // This simulates the stylesheet priorities when injecting using the web extension API.
+        const result = this.popupInjectOuterStylesheetOld(...args);
+
+        const outerStylesheet = Popup.outerStylesheet;
+        const node = document.querySelector('#client-css');
+        if (node !== null && outerStylesheet !== null) {
+            node.parentNode.insertBefore(outerStylesheet, node);
+        }
+
+        return result;
+    }
+
     onWindowResize() {
         if (this.frontend === null) { return; }
         const textSource = this.frontend.textSourceLast;

ext/bg/settings-popup-preview.html

@@ -4,7 +4,7 @@
         <meta charset="UTF-8">
         <meta name="viewport" content="width=device-width,initial-scale=1" />
         <title>Yomichan Popup Preview</title>
-        <link rel="stylesheet" type="text/css" href="/fg/css/client.css">
+        <link rel="stylesheet" type="text/css" href="/fg/css/client.css" id="client-css">
         <style>
             html {
                 transition: background-color 0.25s linear 0s, color 0.25s linear 0s;

ext/bg/settings.html

@@ -259,7 +259,7 @@
                         </div>
                         <div class="col-xs-6">
                             <label for="custom-popup-outer-css">Custom popup outer CSS</label>
-                            <div><textarea autocomplete="off" spellcheck="false" wrap="soft" id="custom-popup-outer-css" class="form-control"></textarea></div>
+                            <div><textarea autocomplete="off" spellcheck="false" wrap="soft" id="custom-popup-outer-css" class="form-control" placeholder="iframe.yomichan-float { /*styles*/ }"></textarea></div>
                         </div>
                     </div>
                 </div>