[gnome-builder/wip/gtk4-port] libide/gui: make notifications button flat

[Christian Hergert <chergert src gnome org>]

commit 29f9f7fe09952c69fd9acf64c045bd185dd06727
Author: Christian Hergert <chergert redhat com>
Date:   Thu Jun 30 15:46:08 2022 -0700

    libide/gui: make notifications button flat
    
    It looks weird when it pops in with the button style.

 src/libide/gui/ide-notifications-button.ui | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/libide/gui/ide-notifications-button.ui b/src/libide/gui/ide-notifications-button.ui
index 0d1cfe230..42933650e 100644
--- a/src/libide/gui/ide-notifications-button.ui
+++ b/src/libide/gui/ide-notifications-button.ui
@@ -8,6 +8,9 @@
         <property name="reveal-child">false</property>
         <child>
           <object class="GtkMenuButton" id="menu_button">
+            <style>
+              <class name="flat"/>
+            </style>
             <property name="popover">
               <object class="GtkPopover" id="popover">
                 <style>