html_composer_message

When you hit the expand arrow in Odoo's chatter and land in the full email composer, the big writing area is not a generic HTML field. It is html_composer_message, a purpose-built subclass of the html_mail widget, which is itself the email-safe variant of the standard html editor. The mail module applies it to the body field of the composer wizard and of scheduled messages.

Its job is to make the editor behave like a message writer rather than a document writer. Reading getConfig() in the source: the video plugin is filtered out, banner commands are disabled in the powerbox, and a mention plugin is added so typing @ suggests people exactly like in Discuss, wired to the thread the composer was opened from. The signature block is tracked separately, so Odoo can tell an empty message with a signature from a message someone actually wrote.

The widget also carries composer-specific plumbing on a shared event bus: it reports whether the body is genuinely empty on an accidental discard, hands the HTML back without the signature when the dialog saves, and deletes inline attachment previews from the body when their chip is removed in the attachment list below the editor.

Nobody configures this widget, but knowing how it behaves saves real support time, because several of its behaviors look like bugs to end users and are in fact rules.

The one that generates the most questions: dynamic placeholders only exist in batch mode. If a user opens the composer for a single customer and cannot find the placeholder command they saw in a template tutorial, that is the widget stripping the placeholder plugins because composition_batch is false. Placeholders like a contact's name only make sense when one email fans out to many recipients; for a single message you would just type the name. Train users on that rule once and the ticket disappears.

Second, the quoted thread in reply-all mode is collapsed behind an expander rather than deleted. Users who think their reply lost the history just need the toggle. Third, images and files pasted into the body become real attachments of the message, not just embedded blobs, so they appear in the attachment list and count for the recipient, which is usually what a business sender wants.

How mail applies it (from the scheduled message form)

<field name="body" widget="html_composer_message" class="h-100"/>

Reusing it on a custom wizard

The widget assumes composer-shaped data. If you borrow it for your own sending wizard, mirror the fields it reads:

body = fields.Html()
model = fields.Char()  # target model of the thread
res_ids = fields.Char()  # JSON list of target ids
composition_batch = fields.Boolean()
composition_comment_option = fields.Selection([...])

With model and res_ids present, mentions and suggestions resolve against the right thread; without them the editor still works but the thread-aware features quietly do nothing.

The most instructive part of the source is the attachment lifecycle, because three components cooperate through env.fullComposerBus.

Paste in, link up. When a file lands in the editor, onAttachmentChange links it into the record's attachment_ids, but only when the record is mail.compose.message and the view loaded attachment_ids. On mail.scheduled.message, which uses the same widget, the hook deliberately does nothing.

Chip out, body clean. When the user removes an attachment chip below the editor, the attachment list widget fires ATTACHMENT_REMOVED, and this widget deletes every element in the body carrying that data-attachment-id, then records a history step so undo works.

Save without the signature. On SAVE_CONTENT, the widget strips .o-signature-container blocks and returns the remaining HTML, plus a flag saying whether a signature block was present. Odoo 18 returned innerText flattened by a whitespace regex here, and the 18 source carries a TODO admitting the regex merges consecutive newlines; 19 replaced that with proper HTML markup. If discarded drafts from the full composer looked mangled to your users on 18, that was why, and upgrading genuinely fixes it.

Odoo 20 is expected at Odoo Experience in Brussels, 24 to 26 September 2026. The notes below read the public development branch, which can still change before release; we re-verify against the shipped version.

The mention machinery is replaced. The composer-specific MentionPlugin file is deleted on master and a new MailFullComposerSuggestionPlugin takes its place, built on the same MentionList and suggestion system Discuss uses, delimiter handling included. Patches against the 19 mention plugin will not survive.

Placeholder plugins are reorganized: the batch-mode filter switches from DYNAMIC_PLACEHOLDER_PLUGINS to a new DYNAMIC_FIELD_PLUGINS set, part of a wider dynamic-field rework in the editor.

Attachment tracking gets an observer. The widget grows a MutationObserver-based attachment set with cleanup on unmount, and the signature class moves to a renamed user_signature_plugin module. All internal, but they are exactly the seams where composer customizations sit, so plan a retest during your Odoo 20 migration.