Add extension code

makexpi.sh

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+#!/bin/sh
+
+NAME=thunderbird-thread-in-same-folder
+VERSION=1.00
+
+ZIPFILE=${NAME}-${VERSION}.xpi
+
+rm -f ${ZIPFILE}
+
+(cd src && zip -r - .) > ${ZIPFILE}

src/background.js

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+'use strict'
+
+async function* iterateMessagePages(page) {
+    for (let message of page.messages) {
+        yield message;
+    }
+
+    while (page.id) {
+        page = await messenger.messages.continueList(page.id);
+        for (let message of page.messages) {
+            yield message;
+        }
+    }
+}
+
+async function load() {
+    await messenger.messages.onNewMailReceived.addListener(async (folder, messages) => {
+        for await (let message of iterateMessagePages(messages)) {
+            let full = await messenger.messages.getFull(message.id);
+            let inReplyTos = full["headers"]["in-reply-to"];
+            if (inReplyTos.length > 0) {
+                let inReplyTo = inReplyTos[0]
+                if (inReplyTo.startsWith("<")) {
+                    inReplyTo = inReplyTo.substring(1);
+                }
+                if (inReplyTo.endsWith(">")) {
+                    inReplyTo = inReplyTo.substring(0, inReplyTo.length - 1)
+                }
+                let queryResult = await messenger.messages.query({"headerMessageId" : inReplyTo})
+                if (queryResult.messages.length > 0) {
+                    let parent = queryResult.messages[0]
+		    if (parent['folder'] != message['folder']) {
+		        messenger.messages.move([message.id], parent['folder'])
+		    }
+                }
+            }
+        }
+    })
+}
+
+document.addEventListener("DOMContentLoaded", load);

src/manifest.json

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+{
+    "name": "Thread in same folder",
+    "description": "Runs after junk classification and filtering to make sure a message lands in the same folder as its parent in a thread",
+    "version": "1.0",
+    "author": "Petros Koutsolampros",
+    "applications": {
+        "gecko": {
+            "id": "threadInSameFolder@pklampros.io",
+            "strict_min_version": "78.0"
+        }
+    },
+    "background": {
+      "scripts": [
+        "background.js"
+      ]
+    },
+    "permissions": [
+        "messagesRead",
+        "accountsRead",
+        "messagesMove"
+    ],
+    "manifest_version": 2
+}