Lossmol – The right direction

A Thunderbird add-on for writing and reading mail in right-to-left scripts (such as Hebrew, Arabic, Syriac, Thaana, N'Ko): paragraphs are displayed according to their dominant language, running right to left and sitting on the right, without fighting the editor. Built on today's Thunderbird extension APIs (Thunderbird 128 and later, Manifest V3). Below, "Hebrew" stands for any of these scripts.

Writing

• Each paragraph takes its direction from its words as you type. Mostly Hebrew or Arabic: right to left. So an English paragraph in a Hebrew message stays left to right, and the other way round. The direction goes into the paragraph's dir attribute, which is what the recipient's mail client sees. A new paragraph opens where the previous one was, so the cursor doesn't jump; its first words then decide. A paragraph still empty when you send carries no direction of its own.
• By hand: the Lossmol button in the compose window's format toolbar switches the current paragraph. Its icon shows the paragraph the cursor is in (lines on the right: right to left; on the left: left to right), and its tooltip says what a click would do. Ctrl + right Shift makes it right to left and Ctrl + left Shift left to right (as in Word and Windows). The editor's right-click menu (and the button's) also has Whole message: right-to-left / left-to-right. A paragraph set by hand stays as you set it.
• Settings (Add-ons Manager → Lossmol → Preferences): the direction a new message starts in; whether paragraphs follow their words at all; whether the display applies when reading (below).

Sending format. Thunderbird's "Automatic" sending format downgrades a message to plain text when it sees no formatting worth keeping, and it never counts a paragraph's direction as formatting, so the direction would be lost on the way. Lossmol therefore switches such a message to "HTML and plain text" just before it's sent (a setting; on by default). If your account composes in plain text to begin with (Account Settings → Composition & Addressing → Compose messages in HTML format unticked), there is no HTML to carry a direction and Lossmol can't help.

Quoted and forwarded messages inside a reply are judged paragraph by paragraph like everything else, but "whole message" leaves them alone.

Reading

Paragraphs are displayed according to their dominant language. Where right-to-left text (such as Hebrew or Arabic) is left-aligned (a common result of Outlook and web forms), it is shown right to left, paragraph by paragraph, so a mixed message comes out right in both directions. Plain-text messages are handled by paragraph too (a paragraph being a run of lines up to a blank one).

The Lossmol button in the message header tells you what you're looking at: in full colour, Lossmol adjusted the display of paragraphs in this message (hover for how many paragraphs); as an outline, the message is displayed the way its own instructions say; grey with left-aligned lines, you switched Lossmol off for this message. Clicking it switches between Lossmol's display and the original display; the next message follows the setting again. Right-click the button for the setting itself ("all messages") and the preferences.

How a paragraph is judged

• Each block by its own text — <p>, <div>, <li>, heading, table cell, and so on; text inside nested blocks is left out. So a wrapper never flips just because the message is mostly Hebrew, and each paragraph gets its own direction.
• Words, not letters. English words run longer and Hebrew leaves out most vowels, so counting letters leans English. A word is a run of letters in one script. Over half the words Hebrew (or Arabic, Syriac, Thaana, N'Ko): right to left.
• Lines that aren't paragraphs — a message written with Enter as a plain line break, or plain text — are grouped into paragraphs and, when they disagree, each paragraph is wrapped in a <div> of its own so it can carry its own direction.
• When reading, the sender's word counts if the sender knows about direction. A message with dir="rtl" anywhere was written with direction in mind (Lossmol, Gmail's or Outlook's right-to-left button), so any dir it puts on a paragraph is deliberate and is kept, even dir="ltr" on Hebrew. A message with no dir="rtl" at all comes from a client that stamps dir="ltr" on everything or writes none (Yahoo, Gmail's wrapper, Outlook); that says nothing, so every paragraph is judged by its words.

Install

From a release: Thunderbird → Add-ons and Themes → gear → Install Add-on From File… → the .xpi.

For development: Add-ons and Themes → gear → Debug Add-ons → Load Temporary Add-on… → this folder's manifest.json. After changing the code: Reload there, then open a new compose window or message.

Files

• manifest.json — Manifest V3; compose_action is the toolbar button.
• background.js — registers the scripts below for compose windows and displayed messages, injects them into windows already open, and wires the button and the context menu to compose.js.
• lib/direction.js — judges a text: rtl, ltr, or nothing to judge by.
• lib/blocks.js — the blocks of a document and their lines: judging, wrapping lines, keeping the caret, watching for changes.
• compose.js — in the editor: automatic direction, the commands, the keyboard shortcuts, the default direction.
• display.js, display.css — in a displayed message.
• options/ — the settings page. _locales/ — English, Hebrew and Arabic (the settings page, the menu and the button follow Thunderbird's language).
• tools/pack.sh — builds dist/lossmol-<version>.xpi. tools/lint.sh — web-ext lint in a one-shot container (podman by default, CONTAINER=docker to use Docker). It warns about the compose and messagesRead permissions only because it checks against Firefox's list; Thunderbird has them.
• test/direction.test.js — node --test test/direction.test.js. test/compose.html, test/display.html — stand-ins for the editor and a displayed message, to try the scripts in any browser (serve the folder over HTTP, e.g. python3 -m http.server, and open them).

Licence

MIT — see LICENSE. Use it freely, as long as the licence text and the credit stay with every copy.