[FIX] mass_mailing: preserve changes on preview templates

Previewing templates in the "SELECT A TEMPLATE" tab of mass_mailing made
one lose all their changes. This makes sure they are preserved and
restored when needed.

task-2778411

closes odoo/odoo#85932

X-original-commit: e96e11d6ab8e5f7a6d5da498409cc0d5fb26cad1
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>
Signed-off-by: Antoine Guenet <age@odoo.com>

addons/mass_mailing/static/src/js/mass_mailing_widget.js

@@ -244,6 +244,8 @@ var MassMailingFieldHtml = FieldHtml.extend({
         }
         this.switchThemeLast = themeParams;
 
+        this.$lastContent = this.$content.find('.o_mail_wrapper_td').contents();
+
         this.$content.closest('body').removeClass(this._allClasses).addClass(themeParams.className);
 
         const old_layout = this.$content.find('.o_layout')[0];
@@ -475,7 +477,11 @@ var MassMailingFieldHtml = FieldHtml.extend({
             self._switchThemes(themeParams);
         });
         $themeSelector.on("mouseleave", ".dropdown-item", function (e) {
-            self._switchThemes(selectedTheme);
+            if (self.$lastContent) {
+                self._switchThemes(Object.assign({}, selectedTheme, {template: self.$lastContent}));
+            } else {
+                self._switchThemes(selectedTheme);
+            }
         });
         $themeSelector.on("click", '[data-toggle="dropdown"]', function (e) {
             var $menu = $themeSelector.find('.dropdown-menu');
@@ -498,6 +504,9 @@ var MassMailingFieldHtml = FieldHtml.extend({
             // Notify form view
             $themeSelector.find('.dropdown-item.selected').removeClass('selected');
             $themeSelector.find('.dropdown-item:eq(' + themesParams.indexOf(selectedTheme) + ')').addClass('selected');
+
+            // Invalidate previous content.
+            self.$lastContent = undefined;
         };
 
         $themeSelector.on("click", ".dropdown-item", selectTheme);