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Changes between Version 3 and Version 4 of Ticket #31484

Ticket #31484
- Property Summary changed from Widgets appearing in Customizer preview during a theme preview may not correspond to controls in Widgets panel to Widgets in Customizer during theme preview fail to preview widgets with prior sidebars_widgets theme mod and fail to apply upon activation

-                      v3
+                      v4
 The problem was introduced in WordPress 4.1, specifically in r29905 for #29983, since we're no longer posting all of the settings from the Customizer pane to the preview for performance reasons: only the changed (dirty) settings get POSTed to the Preview when it is loaded.
+Additionally, when attempting to activate the previous theme that had a cached widgets configuration in the `sidebars_widgets` theme mod, this prior configuration is not getting restored when activating the previously-active theme: the sidebar configuration from the current theme is overwriting the previous theme's configuration. I believe this is due to the `old_sidebars_widgets_data` not being marked as dirty, and so it is not being submitted when saving the settings.
 Steps to reproduce:
 …
 . Live-preview the Twenty Thirteen theme either via the Themes admin page, or via the new Customizer theme previewer
 . Open the Widgets panel and note that the widgets listed are correctly those for Twenty Thirteen, but notice in the Preview that the widgets from Twenty Fourteen are unexpectedly showing.
+. Press *Save & Activate* when redirected to the frontend with the Twenty Thirteen theme active, notice that the Twenty Fourteen widgets are incorrectly shown.
+For previous issues related to widgets in the Customizer being lost during a theme switch, see #27897.