fix: set size of GTK about panel icon (#18957)

docs/api/app.md

@@ -1182,7 +1182,7 @@ Show the app's about panel options. These options can be overridden with `app.se
   * `credits` String (optional) - Credit information. _macOS_
   * `authors` String[] (optional) - List of app authors. _Linux_
   * `website` String (optional) - The app's website. _Linux_
-  * `iconPath` String (optional) - Path to the app's icon. _Linux_
+  * `iconPath` String (optional) - Path to the app's icon. Will be shown as 64x64 pixels while retaining aspect ratio. _Linux_
 
 Set the about panel options. This will override the values defined in the app's
 `.plist` file on MacOS. See the [Apple docs][about-panel-options] for more details. On Linux, values must be set in order to be shown; there are no defaults.

shell/browser/browser_linux.cc

@@ -167,7 +167,12 @@ void Browser::ShowAboutPanel() {
   }
   if ((str = opts.FindStringKey("iconPath"))) {
     GError* error = nullptr;
-    GdkPixbuf* icon = gdk_pixbuf_new_from_file(str->c_str(), &error);
+    constexpr int width = 64;   // width of about panel icon in pixels
+    constexpr int height = 64;  // height of about panel icon in pixels
+
+    // set preserve_aspect_ratio to true
+    GdkPixbuf* icon =
+        gdk_pixbuf_new_from_file_at_size(str->c_str(), width, height, &error);
     if (error != nullptr) {
       g_warning("%s", error->message);
       g_clear_error(&error);