[FIX] mass_mailing: remove overflow on mass mailing container

The overflow: auto on the container in mass mailing generated a
scrollbar whenever the drag and drop of snippets because
of the drop zones.

Task-2781002

X-original-commit: 2af04ad4798e11bb2c08798dddb04ed32c42992e
Part-of: odoo/odoo#88679

addons/web_editor/static/src/js/editor/snippets.editor.js

@@ -2428,6 +2428,9 @@ var SnippetsMenu = Widget.extend({
                     const prom = new Promise(resolve => dragAndDropResolve = () => resolve());
                     self._mutex.exec(() => prom);
 
+                    const doc = self.options.wysiwyg.odooEditor.document;
+                    $(doc.body).addClass('oe_dropzone_active');
+
                     self.options.wysiwyg.odooEditor.automaticStepUnactive();
 
                     self.$el.find('.oe_snippet_thumbnail').addClass('o_we_already_dragging');
@@ -2509,6 +2512,7 @@ var SnippetsMenu = Widget.extend({
                 },
                 stop: async function (ev, ui) {
                     const doc = self.options.wysiwyg.odooEditor.document;
+                    $(doc.body).removeClass('oe_dropzone_active');
                     self.options.wysiwyg.odooEditor.automaticStepUnactive();
                     self.options.wysiwyg.odooEditor.automaticStepSkipStack();
                     $toInsert.removeClass('oe_snippet_body');

addons/web_editor/static/src/scss/wysiwyg_iframe.scss

@@ -32,6 +32,9 @@ body.o_in_iframe {
         flex: 1 1 auto;
         overflow: auto;
     }
+    &.oe_dropzone_active .note-editable {
+        overflow: hidden;
+    }
 
     .iframe-utils-zone {
         display: flex;