Change the application menu fallback icon to the "application-menu-symbolic"
Submitted by Artyom Zorin
When the application icon is not being shown in the Application Menu button in the GtkHeaderBar (after the Application Menu has been moved from the Top Bar in GNOME Shell to the header bar in the application window (by switching the Tweak Tool > Top Bar > Show Application Menu option off for example)), the "process-stop-symbolic" icon is shown. This can be confusing, as using the "application-menu-symbolic" icon would be more obvious in conveying that it is an application menu button rather than a stop button.
The change can be made by replacing the "process-stop-symbolic" link with "application-menu-symbolic" in the gtk/gtkheaderbar.c file.