diff --git a/addons/mail/__manifest__.py b/addons/mail/__manifest__.py
index 838d99efc98..c65d02a5f0f 100644
--- a/addons/mail/__manifest__.py
+++ b/addons/mail/__manifest__.py
@@ -77,6 +77,9 @@
             'web/static/src/webclient/webclient.scss',
             'web/static/src/webclient/webclient_extra.scss',
             'web/static/src/core/utils/*.scss',
+            # depends on BS variables, can't be loaded in assets_primary or assets_secondary
+            'mail/static/src/scss/variables/derived_variables.scss',
+            'mail/static/src/scss/composer.scss',
             # Dependency of notification_group, notification_request, thread_needaction_preview and thread_preview
             'mail/static/src/components/notification_list/notification_list_item.scss',
             'mail/static/src/component_hooks/*.js',
@@ -162,7 +165,6 @@
         'web.assets_qweb': [
             'mail/static/src/xml/*.xml',
             'mail/static/src/components/*/*.xml',
-            'mail/static/src/widgets/*/*.xml',
         ],
     },
     'license': 'LGPL-3',
diff --git a/addons/mail/static/src/components/chatter/chatter.js b/addons/mail/static/src/components/chatter/chatter.js
index ab6dc10fc54..c2bf0669c81 100644
--- a/addons/mail/static/src/components/chatter/chatter.js
+++ b/addons/mail/static/src/components/chatter/chatter.js
@@ -2,7 +2,6 @@
 
 import { useComponentToModel } from '@mail/component_hooks/use_component_to_model';
 import { useRefToModel } from '@mail/component_hooks/use_ref_to_model';
-import { useUpdate } from '@mail/component_hooks/use_update';
 import { registerMessagingComponent } from '@mail/utils/messaging_component';
 import { LegacyComponent } from '@web/legacy/legacy_component';
 
@@ -13,15 +12,10 @@ export class Chatter extends LegacyComponent {
      */
     setup() {
         super.setup();
-        useUpdate({ func: () => this._update() });
         useComponentToModel({ fieldName: 'component' });
         useRefToModel({ fieldName: 'scrollPanelRef', refName: 'scrollPanel' });
     }
 
-    //--------------------------------------------------------------------------
-    // Public
-    //--------------------------------------------------------------------------
-
     /**
      * @returns {Chatter}
      */
@@ -29,26 +23,6 @@ export class Chatter extends LegacyComponent {
         return this.props.record;
     }
 
-    //--------------------------------------------------------------------------
-    // Private
-    //--------------------------------------------------------------------------
-
-    /**
-     * @private
-     */
-    _notifyRendered() {
-        this.trigger('o-chatter-rendered');
-    }
-
-    /**
-     * @private
-     */
-    _update() {
-        if (this.chatter.thread) {
-            this._notifyRendered();
-        }
-    }
-
 }
 
 Object.assign(Chatter, {
diff --git a/addons/mail/static/src/components/web_client_view_attachment_view/web_client_view_attachment_view.js b/addons/mail/static/src/components/web_client_view_attachment_view/web_client_view_attachment_view.js
new file mode 100644
index 00000000000..4603b44602d
--- /dev/null
+++ b/addons/mail/static/src/components/web_client_view_attachment_view/web_client_view_attachment_view.js
@@ -0,0 +1,32 @@
+/** @odoo-module **/
+
+import { useComponentToModel } from '@mail/component_hooks/use_component_to_model';
+import { useRefToModel } from '@mail/component_hooks/use_ref_to_model';
+import { useUpdateToModel } from '@mail/component_hooks/use_update_to_model';
+import { registerMessagingComponent } from '@mail/utils/messaging_component';
+
+import { LegacyComponent } from '@web/legacy/legacy_component';
+
+export class WebClientViewAttachmentView extends LegacyComponent {
+    /**
+     * @override
+     */
+    setup() {
+        useComponentToModel({ fieldName: 'component' });
+        useRefToModel({ fieldName: 'iframeViewerPdfRef', refName: 'iframeViewerPdf' });
+        useUpdateToModel({ methodName: 'onComponentUpdate' });
+    }
+    /**
+     * @returns {WebClientViewAttachmentView}
+     */
+    get webClientViewAttachmentView() {
+        return this.props.record;
+    }
+}
+
+Object.assign(WebClientViewAttachmentView, {
+    props: { record: Object },
+    template: 'mail.WebClientViewAttachmentView',
+});
+
+registerMessagingComponent(WebClientViewAttachmentView);
diff --git a/addons/mail/static/src/widgets/attachment_viewer/attachment_viewer.scss b/addons/mail/static/src/components/web_client_view_attachment_view/web_client_view_attachment_view.scss
similarity index 100%
rename from addons/mail/static/src/widgets/attachment_viewer/attachment_viewer.scss
rename to addons/mail/static/src/components/web_client_view_attachment_view/web_client_view_attachment_view.scss
diff --git a/addons/mail/static/src/components/web_client_view_attachment_view/web_client_view_attachment_view.xml b/addons/mail/static/src/components/web_client_view_attachment_view/web_client_view_attachment_view.xml
new file mode 100644
index 00000000000..ec45d724369
--- /dev/null
+++ b/addons/mail/static/src/components/web_client_view_attachment_view/web_client_view_attachment_view.xml
@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<templates>
+
+    <t t-name="mail.WebClientViewAttachmentView" owl="1">
+        <t t-if="webClientViewAttachmentView.thread.mainAttachment">
+            <t t-if="!webClientViewAttachmentView.thread.mainAttachment.isPdf">
+                <h3 class="mt0 mb8 ps-2 text-muted float-end"><t t-esc="webClientViewAttachmentView.thread.mainAttachment.filename"/></h3>
+            </t>
+            <div t-if="webClientViewAttachmentView.thread.mainAttachment.isImage" class="o_attachment_preview_img">
+                <img id="attachment_img" class="img img-fluid d-block" t-attf-src="/web/image/#{webClientViewAttachmentView.thread.mainAttachment.id}?unique=1"/>
+            </div>
+            <iframe t-if="webClientViewAttachmentView.thread.mainAttachment.isPdf" class="mb48" t-attf-src="/web/static/lib/pdfjs/web/viewer.html?file=/web/content/#{webClientViewAttachmentView.thread.mainAttachment.id}?filename%3D#{window.encodeURIComponent(webClientViewAttachmentView.thread.mainAttachment.name)}" t-ref="iframeViewerPdf"/>
+            <t t-if="webClientViewAttachmentView.thread.attachmentsInWebClientView.length > 1">
+                <a class="arrow o_move_previous text-center" href="#" t-on-click="webClientViewAttachmentView.onClickPrevious">
+                    <span class="fa fa-chevron-left"/>
+                </a>
+                <a class="arrow o_move_next text-center" href="#" t-on-click="webClientViewAttachmentView.onClickNext">
+                    <span class="fa fa-chevron-right"/>
+                </a>
+            </t>
+        </t>
+    </t>
+
+</templates>
diff --git a/addons/mail/static/src/components/web_client_view_attachment_view_container/web_client_view_attachment_view_container.js b/addons/mail/static/src/components/web_client_view_attachment_view_container/web_client_view_attachment_view_container.js
new file mode 100644
index 00000000000..afdabf2e61e
--- /dev/null
+++ b/addons/mail/static/src/components/web_client_view_attachment_view_container/web_client_view_attachment_view_container.js
@@ -0,0 +1,91 @@
+/** @odoo-module **/
+
+import { useModels } from '@mail/component_hooks/use_models';
+// ensure components are registered beforehand.
+import '@mail/components/web_client_view_attachment_view/web_client_view_attachment_view';
+import { replace } from '@mail/model/model_field_command';
+import { getMessagingComponent } from '@mail/utils/messaging_component';
+
+const { Component, onWillDestroy, onWillUpdateProps } = owl;
+
+const getNextId = (function () {
+    let tmpId = 0;
+    return () => {
+        tmpId += 1;
+        return tmpId;
+    };
+})();
+
+/**
+ * This component abstracts attachment view component to its parent, so that it
+ * can be mounted and receive messaging data even when an attachment view
+ * component cannot be created. Indeed, in order to create an attachment view
+ * component, we must create an attachment view record, the latter requiring
+ * messaging to be initialized. The view may attempt to create an attachment
+ * view before messaging has been initialized, so this component delays the
+ * mounting of attachment view until it becomes initialized.
+ */
+export class WebClientViewAttachmentViewContainer extends Component {
+
+    /**
+     * @override
+     */
+    setup() {
+        useModels();
+        super.setup();
+        this.webClientViewAttachmentView = undefined;
+        this.webClientViewAttachmentViewId = getNextId();
+        this._insertFromProps(this.props);
+        onWillUpdateProps(nextProps => this._insertFromProps(nextProps));
+        onWillDestroy(() => this._deleteRecord());
+    }
+
+    /**
+     * @private
+     */
+    _deleteRecord() {
+        if (this.webClientViewAttachmentView) {
+            if (this.webClientViewAttachmentView.exists()) {
+                this.webClientViewAttachmentView.delete();
+            }
+            this.webClientViewAttachmentView = undefined;
+        }
+    }
+
+    /**
+     * @private
+     */
+    async _insertFromProps(props) {
+        const messaging = await this.env.services.messaging.get();
+        if (this.__owl__.status === 5 /* destroyed */) {
+            this._deleteRecord();
+            return;
+        }
+        if (!props.threadId) {
+            this._deleteRecord();
+            return;
+        }
+        const thread = messaging.models['Thread'].insert({
+            id: props.threadId,
+            model: props.threadModel,
+        });
+        this.webClientViewAttachmentView = messaging.models['WebClientViewAttachmentView'].insert({
+            id: this.webClientViewAttachmentViewId,
+            thread: replace(thread),
+        });
+        this.render();
+    }
+
+}
+
+Object.assign(WebClientViewAttachmentViewContainer, {
+    components: { WebClientViewAttachmentView: getMessagingComponent('WebClientViewAttachmentView') },
+    props: {
+        threadId: {
+            type: Number,
+            optional: true,
+        },
+        threadModel: String,
+    },
+    template: 'mail.WebClientViewAttachmentViewContainer',
+});
diff --git a/addons/mail/static/src/components/web_client_view_attachment_view_container/web_client_view_attachment_view_container.xml b/addons/mail/static/src/components/web_client_view_attachment_view_container/web_client_view_attachment_view_container.xml
new file mode 100644
index 00000000000..3efb6449260
--- /dev/null
+++ b/addons/mail/static/src/components/web_client_view_attachment_view_container/web_client_view_attachment_view_container.xml
@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<templates xml:space="preserve">
+
+    <t t-name="mail.WebClientViewAttachmentViewContainer" owl="1">
+        <t t-if="webClientViewAttachmentView and webClientViewAttachmentView.thread.attachmentsInWebClientView.length > 0">
+            <div class="o_attachment_preview_container">
+                <WebClientViewAttachmentView record="webClientViewAttachmentView"/>
+            </div>
+        </t>
+    </t>
+
+</templates>
diff --git a/addons/mail/static/src/models/thread.js b/addons/mail/static/src/models/thread.js
index ccbd04627f5..b2bc43ee74d 100644
--- a/addons/mail/static/src/models/thread.js
+++ b/addons/mail/static/src/models/thread.js
@@ -579,6 +579,7 @@ registerModel({
                 });
             }
             this.update(values);
+            this.messaging.messagingBus.trigger('o-thread-loaded-data', { thread: this });
         },
         /**
          * Add current user to provided thread's followers.
@@ -949,6 +950,17 @@ registerModel({
                 kwargs: { name: newName },
             });
         },
+        /**
+         * @param {Attachment} attachment
+         */
+        async setMainAttachment(attachment) {
+            this.update({ mainAttachment: replace(attachment) });
+            await this.messaging.rpc({
+                model: 'ir.attachment',
+                method: 'register_as_main_attachment',
+                args: [[this.mainAttachment.id]],
+            });
+        },
         /**
          * Unfollow current partner from this thread.
          */
diff --git a/addons/mail/static/src/models/web_client_view_attachment_view.js b/addons/mail/static/src/models/web_client_view_attachment_view.js
new file mode 100644
index 00000000000..605b3e7a9d1
--- /dev/null
+++ b/addons/mail/static/src/models/web_client_view_attachment_view.js
@@ -0,0 +1,64 @@
+/** @odoo-module **/
+
+import { registerModel } from '@mail/model/model_core';
+import { attr, one } from '@mail/model/model_field';
+import { OnChange } from '@mail/model/model_onchange';
+
+import { hidePDFJSButtons } from '@web/legacy/js/libs/pdfjs';
+
+registerModel({
+    name: 'WebClientViewAttachmentView',
+    identifyingFields: ['id'],
+    recordMethods: {
+        /**
+         * @param {MouseEvent} ev
+         */
+        onClickNext(ev) {
+            ev.preventDefault();
+            const index = this.thread.attachmentsInWebClientView.findIndex(attachment => attachment === this.thread.mainAttachment);
+            this.setMainAttachmentFromIndex(index === this.thread.attachmentsInWebClientView.length - 1 ? 0 : index + 1);
+        },
+        /**
+         * @param {MouseEvent} ev
+         */
+        onClickPrevious(ev) {
+            ev.preventDefault();
+            const index = this.thread.attachmentsInWebClientView.findIndex(attachment => attachment === this.thread.mainAttachment);
+            this.setMainAttachmentFromIndex(index === 0 ? this.thread.attachmentsInWebClientView.length - 1 : index - 1);
+        },
+        onComponentUpdate() {
+            if (this.iframeViewerPdfRef.el) {
+                hidePDFJSButtons(this.iframeViewerPdfRef.el);
+            }
+            this.component.trigger('preview_attachment_validation');
+        },
+        async setMainAttachmentFromIndex(index) {
+            await this.thread.setMainAttachment(this.thread.attachmentsInWebClientView[index]);
+        },
+        /**
+         * @private
+         */
+        _onChangeThreadAttachmentsInWebClientView() {
+            if (!this.thread.mainAttachment && this.thread.attachmentsInWebClientView.length > 0) {
+                this.setMainAttachmentFromIndex(0);
+            }
+        },
+    },
+    fields: {
+        component: attr(),
+        id: attr({
+            readonly: true,
+            required: true,
+        }),
+        iframeViewerPdfRef: attr(),
+        thread: one('Thread', {
+            required: true,
+        }),
+    },
+    onChanges: [
+        new OnChange({
+            dependencies: ['thread.attachmentsInWebClientView'],
+            methodName: '_onChangeThreadAttachmentsInWebClientView',
+        }),
+    ],
+});
diff --git a/addons/mail/static/src/widgets/attachment_viewer/attachment_viewer.js b/addons/mail/static/src/widgets/attachment_viewer/attachment_viewer.js
deleted file mode 100644
index 9f8ae7617be..00000000000
--- a/addons/mail/static/src/widgets/attachment_viewer/attachment_viewer.js
+++ /dev/null
@@ -1,123 +0,0 @@
-/** @odoo-module **/
-
-import { replace } from '@mail/model/model_field_command';
-
-import core from 'web.core';
-import Widget from 'web.Widget';
-import { hidePDFJSButtons } from '@web/legacy/js/libs/pdfjs';
-
-const QWeb = core.qweb;
-
-export const AttachmentViewer = Widget.extend({
-    className: 'o_attachment_preview_container',
-    events: {
-        'click .arrow.o_move_next': '_onClickNext',
-        'click .arrow.o_move_previous': '_onClickPrevious',
-    },
-    /**
-     * @constructor
-     * @override
-     * @param {Widget} parent
-     * @param {Thread} thread
-     */
-    init(parent, thread) {
-        this._super.apply(this, arguments);
-        this.thread = thread;
-        this.attachments = this.thread.attachmentsInWebClientView;
-        this._setActive();
-    },
-    /**
-     * Render attachment.
-     *
-     * @override
-     */
-    start() {
-        this._renderAttachment();
-        return this._super.apply(this, arguments);
-    },
-
-    //--------------------------------------------------------------------------
-    // Public
-    //--------------------------------------------------------------------------
-
-    /**
-     * Update attachments list and activeAttachment.
-
-     * @param {Thread} thread
-     */
-    updateContents(thread) {
-        this.thread = thread;
-        this.attachments = this.thread.attachmentsInWebClientView;
-        this._setActive();
-        this._renderAttachment();
-    },
-
-    //--------------------------------------------------------------------------
-    // Private
-    //--------------------------------------------------------------------------
-
-    /**
-     * Render template
-     *
-     * @private
-     */
-    _renderAttachment() {
-        this.$el.empty();
-        this.$el.append(QWeb.render('mail.widgets.AttachmentViewer', { thread: this.thread }));
-        if (this.thread.mainAttachment && this.thread.mainAttachment.isPdf) {
-            hidePDFJSButtons(this.el);
-        }
-    },
-
-    /**
-     * @private
-     */
-    _setActive() {
-        if (!this.thread.mainAttachment && this.thread.attachmentsInWebClientView.length > 0) {
-            this._switch_main_attachment(0);
-        }
-    },
-
-    //--------------------------------------------------------------------------
-    // Handlers
-    //--------------------------------------------------------------------------
-
-    /**
-    * Sets the attachment at position index as the new main attachment of
-    * the related model, and display it.
-    **/
-    async _switch_main_attachment(index) {
-        this.thread.update({ mainAttachment: replace(this.thread.attachmentsInWebClientView[index]) });
-        await this._rpc({
-            model: 'ir.attachment',
-            method: 'register_as_main_attachment',
-            args: [[this.thread.mainAttachment['id']]],
-        });
-        this._renderAttachment();
-    },
-
-    /**
-     * On click move to next attachment.
-     *
-     * @private
-     * @param {MouseEvent} ev
-     */
-    _onClickNext(ev) {
-        ev.preventDefault();
-        var index = _.findIndex(this.thread.attachmentsInWebClientView, this.thread.mainAttachment);
-        index = index === this.thread.attachmentsInWebClientView.length - 1 ? 0 : index + 1;
-        this._switch_main_attachment(index);
-    },
-    /**
-     * On click move to previous attachment.
-     *
-     * @private
-     * @param {MouseEvent} ev
-     */
-    _onClickPrevious(ev) {
-        ev.preventDefault();
-        var index = _.findIndex(this.thread.attachmentsInWebClientView, this.thread.mainAttachment);
-        index = index === 0 ? this.thread.attachmentsInWebClientView.length - 1 : index - 1;
-        this._switch_main_attachment(index);
-    },
-});
diff --git a/addons/mail/static/src/widgets/attachment_viewer/attachment_viewer.xml b/addons/mail/static/src/widgets/attachment_viewer/attachment_viewer.xml
deleted file mode 100644
index fd017ca00a8..00000000000
--- a/addons/mail/static/src/widgets/attachment_viewer/attachment_viewer.xml
+++ /dev/null
@@ -1,24 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<templates>
-
-    <t t-name="mail.widgets.AttachmentViewer">
-        <t t-if="thread.mainAttachment">
-            <t t-if="!thread.mainAttachment.isPdf">
-                <h3 class="mt0 mb8 ps-2 text-muted float-end"><t t-esc="thread.mainAttachment.filename"/></h3>
-            </t>
-            <div t-if="thread.mainAttachment.isImage" class="o_attachment_preview_img">
-                <img id="attachment_img" class="img img-fluid d-block" t-attf-src="/web/image/#{thread.mainAttachment.id}?unique=1"/>
-            </div>
-            <iframe t-if="thread.mainAttachment.isPdf" class="mb48" t-attf-src="/web/static/lib/pdfjs/web/viewer.html?file=/web/content/#{thread.mainAttachment.id}?filename%3D#{window.encodeURIComponent(thread.mainAttachment.name)}"/>
-            <t t-if="thread.attachmentsInWebClientView.length > 1">
-                <a class="arrow o_move_previous text-center" href="#">
-                    <span class="fa fa-chevron-left"/>
-                </a>
-                <a class="arrow o_move_next text-center" href="#">
-                    <span class="fa fa-chevron-right"/>
-                </a>
-            </t>
-        </t>
-    </t>
-
-</templates>
diff --git a/addons/mail/static/src/widgets/form_renderer/form_renderer.js b/addons/mail/static/src/widgets/form_renderer/form_renderer.js
index 2c4929eee9c..9bbf6ac9057 100644
--- a/addons/mail/static/src/widgets/form_renderer/form_renderer.js
+++ b/addons/mail/static/src/widgets/form_renderer/form_renderer.js
@@ -1,7 +1,8 @@
 /** @odoo-module **/
 
 import { ChatterContainer } from '@mail/components/chatter_container/chatter_container';
-import { AttachmentViewer } from '@mail/widgets/attachment_viewer/attachment_viewer';
+import { WebClientViewAttachmentViewContainer } from '@mail/components/web_client_view_attachment_view_container/web_client_view_attachment_view_container';
+import { Listener } from '@mail/model/model_listener';
 
 import dom from 'web.dom';
 import FormRenderer from 'web.FormRenderer';
@@ -23,6 +24,10 @@ FormRenderer.include({
      */
     init(parent, state, params) {
         this._super(...arguments);
+        this.modelsListener = new Listener({
+            name: 'interchangeChatter',
+            onChange: () => this._interchangeChatter(),
+        });
         this.hasChatter = params.hasChatter && !params.isFromFormViewDialog;
         this.isChatterInSheet = false;
         this.hasAttachmentViewerFeature = params.hasAttachmentViewerFeature;
@@ -49,14 +54,15 @@ FormRenderer.include({
          * when applying the rendering into the DOM.
          */
         this.chatterContainerTargetPlaceholder = undefined;
-        this.attachmentViewer = undefined;
+        this.webClientViewAttachmentViewContainer = undefined;
         this.attachmentViewerTarget = undefined;
         if (this.hasAttachmentViewerFeature) {
             this.attachmentViewerTarget = document.createElement("div");
             this.attachmentViewerTarget.classList.add("o_attachment_preview");
+            this.webClientViewAttachmentViewContainer = new ComponentWrapper(this, WebClientViewAttachmentViewContainer, this._makeWebClientViewAttachmentViewContainerProps());
+            this.webClientViewAttachmentViewContainer.mount(this.attachmentViewerTarget);
         }
         this.attachmentViewerTargetPlaceholder = undefined;
-        this.on('o_chatter_rendered', this, ev => this._onChatterRendered(ev));
     },
     async initChatter() {
         this._chatterContainerComponent = new ChatterContainerWrapperComponent(
@@ -99,6 +105,9 @@ FormRenderer.include({
         if (this.hasChatter) {
             await this._updateChatterContainerComponent();
         }
+        if (this.hasAttachmentViewer()) {
+            await this._updateWebClientViewAttachmentViewContainer();
+        }
     },
     /**
      * The last rendering of the form view has just been applied into the DOM,
@@ -135,8 +144,10 @@ FormRenderer.include({
      */
     destroy() {
         this._super(...arguments);
+        if (owl.Component.env.services.messaging) {
+            owl.Component.env.services.messaging.modelManager.removeListener(this.modelsListener);
+        }
         this._chatterContainerComponent = undefined;
-        this.off('o_chatter_rendered', this);
     },
 
     //--------------------------------------------------------------------------
@@ -176,10 +187,13 @@ FormRenderer.include({
             }
             $(this._chatterContainerTarget).insertAfter($sheetBg);
         }
-        if (this.hasAttachmentViewer()) {
-            $(this.attachmentViewerTarget).insertAfter($sheetBg);
-        } else {
-            dom.append($sheetBg, $(this.attachmentViewerTarget));
+        if (this.hasAttachmentViewerFeature) {
+            if (this.hasAttachmentViewer()) {
+                $(this.attachmentViewerTarget).insertAfter($sheetBg);
+                this._updateWebClientViewAttachmentViewContainer();
+            } else {
+                this.attachmentViewerTarget.remove();
+            }
         }
         this._updateChatterContainerComponent();
     },
@@ -211,55 +225,24 @@ FormRenderer.include({
             threadModel: this.state.model,
         };
     },
+    _makeWebClientViewAttachmentViewContainerProps() {
+        return {
+            threadId: this.state.res_id,
+            threadModel: this.state.model,
+        };
+    },
     /**
      * @private
      */
     async _updateChatterContainerComponent() {
         const props = this._makeChatterContainerProps();
-        try {
-            await this._chatterContainerComponent.update(props);
-        } catch (error) {
-            if (error.message !== "Mounting operation cancelled") {
-                throw error;
-            }
-        }
+        await this._chatterContainerComponent.update(props);
     },
     /**
      * @private
-     * @param {OdooEvent} ev
-     * @param {Object} ev.data
-     * @param {Attachment[]} ev.data.attachments
-     * @param {Thread} ev.data.thread
      */
-    _onChatterRendered(ev) {
-        if (!this.hasAttachmentViewer()) {
-            if (this.attachmentViewer) {
-                this.attachmentViewer.destroy();
-                this.attachmentViewer = undefined;
-                this._interchangeChatter();
-            }
-            return;
-        }
-        var self = this;
-        const thread = this.messaging.models['Thread'].insert({
-            id: this.state.res_id,
-            model: this.state.model,
-        });
-        if (this.attachmentViewer) {
-            // FIXME should be improved : what if somehow an attachment is replaced in a thread ?
-            if (
-                this.attachmentViewer.thread !== thread ||
-                this.attachmentViewer.attachments.length !== thread.attachmentsInWebClientView.length
-            ) {
-                this.attachmentViewer.updateContents(thread);
-            }
-            this.trigger_up('preview_attachment_validation');
-        } else {
-            this.attachmentViewer = new AttachmentViewer(this, thread);
-            this.attachmentViewer.appendTo($(this.attachmentViewerTarget)).then(function () {
-                self.trigger_up('preview_attachment_validation');
-                self._interchangeChatter();
-            });
-        }
+    async _updateWebClientViewAttachmentViewContainer() {
+        const props = this._makeWebClientViewAttachmentViewContainerProps();
+        await this.webClientViewAttachmentViewContainer.update(props);
     },
 });
diff --git a/addons/mail/static/tests/helpers/mock_server.js b/addons/mail/static/tests/helpers/mock_server.js
index 17e4c374b74..2b78f9c1235 100644
--- a/addons/mail/static/tests/helpers/mock_server.js
+++ b/addons/mail/static/tests/helpers/mock_server.js
@@ -125,6 +125,10 @@ patch(MockServer.prototype, 'mail', {
         if (new RegExp('/mail/channel/\\d+/partner/\\d+/avatar_128').test(route)) {
             return;
         }
+        if (args.model === 'ir.attachment' && args.method === 'register_as_main_attachment') {
+            const ids = args.args[0];
+            return this._mockIrAttachmentRegisterAsMainAttachment(ids);
+        }
         // mail.activity methods
         if (args.model === 'mail.activity' && args.method === 'action_feedback') {
             const ids = args.args[0];
@@ -528,7 +532,7 @@ patch(MockServer.prototype, 'mail', {
                 [['res_id', '=', thread.id], ['res_model', '=', thread_model]],
             ); // order not done for simplicity
             res['attachments'] = this._mockIrAttachment_attachmentFormat(attachments.map(attachment => attachment.id));
-            res['mainAttachment'] = thread.message_main_attachment_id ? [['insert-and-replace', { 'id': thread.message_main_attachment_id }]] : [['clear']];
+            res['mainAttachment'] = thread.message_main_attachment_id ? [['insert-and-replace', { 'id': thread.message_main_attachment_id[0] }]] : [['clear']];
         }
         if (request_list.includes('followers')) {
             const followers = this.pyEnv['mail.followers'].searchRead([['id', 'in', thread.message_follower_ids || []]]);
@@ -613,6 +617,31 @@ patch(MockServer.prototype, 'mail', {
             return res;
         });
     },
+    /**
+     * Simulates `register_as_main_attachment` on `ir.attachment`.
+     *
+     * @private
+     * @param {integer} ids
+     * @param {boolean} [force=true]
+     * @returns {boolean} dummy value for mock server
+     */
+    _mockIrAttachmentRegisterAsMainAttachment(ids, force = true) {
+        const [attachment] = this.getRecords('ir.attachment', [['id', 'in', ids]]);
+        if (!attachment.res_model) {
+            return true; // dummy value for mock server
+        }
+        if (!this.models[attachment.res_model].fields['message_main_attachment_id']) {
+            return true; // dummy value for mock server
+        }
+        const [record] = this.pyEnv[attachment.res_model].searchRead([['id', '=', attachment.res_id]]);
+        if (force || !record.message_main_attachment_id) {
+            this.pyEnv[attachment.res_model].write(
+                [record.id],
+                { message_main_attachment_id: attachment.id },
+            );
+        }
+        return true; // dummy value for mock server
+    },
     /**
      * Simulates `_action_done` on `mail.activity`.
      *
diff --git a/addons/mail/static/tests/helpers/test_utils.js b/addons/mail/static/tests/helpers/test_utils.js
index 45dd7c30b1a..435108523cf 100644
--- a/addons/mail/static/tests/helpers/test_utils.js
+++ b/addons/mail/static/tests/helpers/test_utils.js
@@ -446,25 +446,44 @@ function getOpenDiscuss(afterEvent, webClient, { context = {}, params, ...props
 }
 
 function getOpenFormView(afterEvent, openView) {
-    return async function openFormView(action, { props, waitUntilMessagesLoaded = true } = {}) {
+    return async function openFormView(action, { props, waitUntilDataLoaded = true, waitUntilMessagesLoaded = true } = {}) {
         action['views'] = [[false, 'form']];
-        if (waitUntilMessagesLoaded) {
-            return afterNextRender(() => afterEvent({
-                eventName: 'o-thread-view-hint-processed',
-                func: () => openView(action, props),
-                message: "should wait until chatter loaded its messages",
-                predicate: ({ hint, threadViewer }) => {
-                    return (
-                        hint.type === 'messages-loaded' &&
-                        threadViewer &&
-                        threadViewer.thread.model === action.res_model &&
-                        threadViewer.thread.id === action.res_id
-                    )
-                },
-            }));
+        const func = () => openView(action, props);
+        const waitData = func => afterNextRender(() => afterEvent({
+            eventName: 'o-thread-loaded-data',
+            func,
+            message: "should wait until chatter loaded its data",
+            predicate: ({ thread }) => {
+                return (
+                    thread.model === action.res_model &&
+                    thread.id === action.res_id
+                );
+            },
+        }));
+        const waitMessages = func => afterNextRender(() => afterEvent({
+            eventName: 'o-thread-view-hint-processed',
+            func,
+            message: "should wait until chatter loaded its messages",
+            predicate: ({ hint, threadViewer }) => {
+                return (
+                    hint.type === 'messages-loaded' &&
+                    threadViewer &&
+                    threadViewer.thread.model === action.res_model &&
+                    threadViewer.thread.id === action.res_id
+                );
+            },
+        }));
+        if (waitUntilDataLoaded && waitUntilMessagesLoaded) {
+            return waitData(() => waitMessages(func));
         }
-        return openView(action, props);
-    }
+        if (waitUntilDataLoaded) {
+            return waitData(func);
+        }
+        if (waitUntilMessagesLoaded) {
+            return waitMessages(func);
+        }
+        return func();
+    };
 }
 
 //------------------------------------------------------------------------------