Fix dropdown delegating and some UI problems (#34014)

The old logic is incomplete. See the comment for the improved logic.

Fix #34011

And more fixes:

1. use empty "alt" for images, otherwise the width is not right when the
image fails to load
2. remove the "dropdown icon" patch, because it has been clearly done in
"dropdown.js" now
3. remove the "dropdown filtered item" patch, added a clear callback,
and improve the logic
4. fix global init when a node is removed and added back gain (eg: the
"cherry pick" dialog with a dropdown)

modules/templates/util_avatar.go

@@ -34,7 +34,8 @@ func AvatarHTML(src string, size int, class, name string) template.HTML {
 		name = "avatar"
 	}
 
-	return template.HTML(`<img loading="lazy" class="` + class + `" src="` + src + `" title="` + html.EscapeString(name) + `" width="` + sizeStr + `" height="` + sizeStr + `"/>`)
+	// use empty alt, otherwise if the image fails to load, the width will follow the "alt" text's width
+	return template.HTML(`<img loading="lazy" alt="" class="` + class + `" src="` + src + `" title="` + html.EscapeString(name) + `" width="` + sizeStr + `" height="` + sizeStr + `"/>`)
 }
 
 // Avatar renders user avatars. args: user, size (int), class (string)