[2.2.x] Fixed #30153 -- Fixed incorrect form Media asset ordering after three way merge.

Delaying merging assets as long as possible avoids introducing
incorrect relative orderings that cause a broken final result.
Backport of 959d0c078a from master.

django/forms/widgets.py

@@ -48,8 +48,8 @@ class Media:
                 css = {}
             if js is None:
                 js = []
-        self._css = css
-        self._js = js
+        self._css_lists = [css]
+        self._js_lists = [js]
 
     def __repr__(self):
         return 'Media(css=%r, js=%r)' % (self._css, self._js)
@@ -57,6 +57,25 @@ class Media:
     def __str__(self):
         return self.render()
 
+    @property
+    def _css(self):
+        css = self._css_lists[0]
+        # filter(None, ...) avoids calling merge with empty dicts.
+        for obj in filter(None, self._css_lists[1:]):
+            css = {
+                medium: self.merge(css.get(medium, []), obj.get(medium, []))
+                for medium in css.keys() | obj.keys()
+            }
+        return css
+
+    @property
+    def _js(self):
+        js = self._js_lists[0]
+        # filter(None, ...) avoids calling merge() with empty lists.
+        for obj in filter(None, self._js_lists[1:]):
+            js = self.merge(js, obj)
+        return js
+
     def render(self):
         return mark_safe('\n'.join(chain.from_iterable(getattr(self, 'render_' + name)() for name in MEDIA_TYPES)))
 
@@ -132,11 +151,8 @@ class Media:
 
     def __add__(self, other):
         combined = Media()
-        combined._js = self.merge(self._js, other._js)
-        combined._css = {
-            medium: self.merge(self._css.get(medium, []), other._css.get(medium, []))
-            for medium in self._css.keys() | other._css.keys()
-        }
+        combined._css_lists = self._css_lists + other._css_lists
+        combined._js_lists = self._js_lists + other._js_lists
         return combined