Avoid needlessly updating stylesheets (#5278)

panel/theme/base.py

@@ -117,7 +117,7 @@ def __init__(self, theme=None, **params):
 
     def _reapply(
         self, viewable: Viewable, root: Model, old_models: List[Model] = None,
-        isolated: bool=True, cache=None
+        isolated: bool=True, cache=None, document=None
     ) -> None:
         ref = root.ref['id']
         for o in viewable.select():
@@ -129,7 +129,7 @@ def _reapply(
             if old_models and ref in o._models:
                 if o._models[ref][0] in old_models:
                     continue
-            self._apply_modifiers(o, ref, self.theme, isolated, cache)
+            self._apply_modifiers(o, ref, self.theme, isolated, cache, document)
 
     def _apply_hooks(self, viewable: Viewable, root: Model, changed: Viewable, old_models=None) -> None:
         from ..io.state import state
@@ -138,7 +138,7 @@ def _apply_hooks(self, viewable: Viewable, root: Model, changed: Viewable, old_m
         else:
             state._stylesheets[root.document] = cache = {}
         with root.document.models.freeze():
-            self._reapply(changed, root, old_models, isolated=False, cache=cache)
+            self._reapply(changed, root, old_models, isolated=False, cache=cache, document=root.document)
 
     def _wrapper(self, viewable):
         return viewable
@@ -222,20 +222,23 @@ def _patch_modifiers(cls, doc, modifiers, cache):
             modifiers['stylesheets'] = stylesheets
 
     @classmethod
-    def _apply_modifiers(cls, viewable: Viewable, mref: str, theme: Theme, isolated: bool, cache={}) -> None:
+    def _apply_modifiers(
+        cls, viewable: Viewable, mref: str, theme: Theme, isolated: bool,
+        cache={}, document=None
+    ) -> None:
         if mref not in viewable._models:
             return
         model, _ = viewable._models[mref]
         modifiers, child_modifiers = cls._get_modifiers(viewable, theme, isolated)
-        cls._patch_modifiers(model.document, modifiers, cache)
+        cls._patch_modifiers(model.document or document, modifiers, cache)
         if child_modifiers:
             for child in viewable:
-                cls._apply_params(child, mref, child_modifiers)
+                cls._apply_params(child, mref, child_modifiers, document)
         if modifiers:
-            cls._apply_params(viewable, mref, modifiers)
+            cls._apply_params(viewable, mref, modifiers, document)
 
     @classmethod
-    def _apply_params(cls, viewable, mref, modifiers):
+    def _apply_params(cls, viewable, mref, modifiers, document=None):
         # Apply params never sync the modifier values with the Viewable
         # This should not be a concern since most `Layoutable` properties,
         # e.g. stylesheets or sizing_mode, are not synced between the
@@ -251,14 +254,33 @@ def _apply_params(cls, viewable, mref, modifiers):
         if 'stylesheets' in modifiers:
             params['stylesheets'] = modifiers['stylesheets'] + viewable.stylesheets
         props = viewable._process_param_change(params)
-        doc = model.document
+        doc = model.document or document
         if doc and 'dist_url' in doc._template_variables:
             dist_url = doc._template_variables['dist_url']
         else:
             dist_url = CDN_DIST
         for stylesheet in props.get('stylesheets', []):
             if isinstance(stylesheet, ImportedStyleSheet):
                 patch_stylesheet(stylesheet, dist_url)
+
+        # Do not update stylesheets if they match
+        if 'stylesheets' in props and len(model.stylesheets) == len(props['stylesheets']):
+            all_match = True
+            stylesheets = []
+            for st1, st2 in zip(model.stylesheets, props['stylesheets']):
+                if st1 == st2:
+                    stylesheets.append(st1)
+                    continue
+                elif type(st1) is type(st2) and isinstance(st1, ImportedStyleSheet) and st1.url == st2.url:
+                    stylesheets.append(st1)
+                    continue
+                stylesheets.append(st2)
+                all_match = False
+            if all_match:
+                del props['stylesheets']
+            else:
+                props['stylesheets'] = stylesheets
+
         model.update(**props)
         if hasattr(viewable, '_synced_properties') and 'objects' in viewable._property_mapping:
             obj_key = viewable._property_mapping['objects']