Room marked as unread when I was the last person to speak

This issue has been migrated from [#14837](https://github.com/matrix-org/synapse/issues/14837).

---

### Description

I have a room on matrix.org where the sync response shows me as the last person to send an event, but also has `notification_count: 1`. @gsouquet tells me that should never happen: if I sent the last event, the room should not be unread.

```
"unread_notifications": {
"notification_count": 1,
"highlight_count": 0
},
```

The room ID is `!KCcFbqJHMQscdVsFTa:element.io`

The URL I used to sync was: `https://matrix-client.matrix.org/_matrix/client/r0/sync?filter=%7B%22room%22%3A%7B%22timeline%22%3A%7B%22unread_thread_notifications%22%3Atrue%2C%22limit%22%3A20%7D%2C%22state%22%3A%7B%22lazy_load_members%22%3Atrue%7D%7D%7D&timeout=0&_cacheBuster=1673621386081`

My username is `@andybalaam:matrix.org`

The full sync response for this room is:

```
"!KCcFbqJHMQscdVsFTa:element.io": {
"timeline": {
"events": [
{
"content": {
"creator": "@lgian:element.io",
"room_version": "9"
},
"origin_server_ts": 1669207170409,
"sender": "@lgian:element.io",
"state_key": "",
"type": "m.room.create",
"unsigned": {
"age": 1651989
},
"event_id": "$T4yQFiEjK1NwvniRWmyjzdJQeJwSvz2TVh55BbPuhw8"
},
{
"content": {
"avatar_url": "mxc://element.io/079b96b843c4fe3f75712533f31233038d0d53d8",
"displayname": "lgian",
"membership": "join"
},
"origin_server_ts": 1669207170696,
"sender": "@lgian:element.io",
"state_key": "@lgian:element.io",
"type": "m.room.member",
"unsigned": {
"age": 1651702
},
"event_id": "$STEseiYuCwtuJgj2nJH4_l2mq-b7aZJGMXefty9Yki4"
},
{
"content": {
"ban": 50,
"events": {
"m.room.avatar": 50,
"m.room.canonical_alias": 50,
"m.room.encryption": 100,
"m.room.history_visibility": 100,
"m.room.name": 50,
"m.room.power_levels": 100,
"m.room.server_acl": 100,
"m.room.tombstone": 100
},
"events_default": 0,
"historical": 100,
"invite": 0,
"kick": 50,
"redact": 50,
"state_default": 50,
"users": {
"@andybalaam:matrix.org": 100,
"@lgian:element.io": 100
},
"users_default": 0
},
"origin_server_ts": 1669207170782,
"sender": "@lgian:element.io",
"state_key": "",
"type": "m.room.power_levels",
"unsigned": {
"age": 1651616
},
"event_id": "$DoEVRqa-hHhg1GJdnlT7X_BPXgWpvv3ACp9akZ6pjc8"
},
{
"content": {
"join_rule": "invite"
},
"origin_server_ts": 1669207170814,
"sender": "@lgian:element.io",
"state_key": "",
"type": "m.room.join_rules",
"unsigned": {
"age": 1651584
},
"event_id": "$5XX9KB-PNpUmp_zphOKtNVRdSiPPDwKO-zo1TWrsQDQ"
},
{
"content": {
"history_visibility": "shared"
},
"origin_server_ts": 1669207170820,
"sender": "@lgian:element.io",
"state_key": "",
"type": "m.room.history_visibility",
"unsigned": {
"age": 1651578
},
"event_id": "$AoRJeIFLou1NpTPo8ydqXFV-5LyeXCDnT8NhJfot7nY"
},
{
"content": {
"guest_access": "can_join"
},
"origin_server_ts": 1669207170829,
"sender": "@lgian:element.io",
"state_key": "",
"type": "m.room.guest_access",
"unsigned": {
"age": 1651569
},
"event_id": "$10LzQqIAeVJd6m5jbQ5iFZuIdUmNcCnj0k3-Yqkl6GE"
},
{
"content": {
"algorithm": "m.megolm.v1.aes-sha2"
},
"origin_server_ts": 1669207170837,
"sender": "@lgian:element.io",
"state_key": "",
"type": "m.room.encryption",
"unsigned": {
"age": 1651561
},
"event_id": "$ZGp0VZGbACUCESNl8F7k2GM_w3umBFrhzPD17eR834Q"
},
{
"content": {
"avatar_url": "mxc://matrix.org/yqsMYtlqLZIlMOnqBRAXnkUm",
"displayname": "andybalaam",
"is_direct": true,
"membership": "invite"
},
"origin_server_ts": 1669207171121,
"sender": "@lgian:element.io",
"state_key": "@andybalaam:matrix.org",
"type": "m.room.member",
"unsigned": {
"age": 22
},
"event_id": "$rWoSNtthPJ1WHmoO_qqvX-Q-Jmy9dTqKfKZFTw4idPg"
},
{
"content": {
"algorithm": "m.megolm.v1.aes-sha2",
"ciphertext": "AwgAEuABEy11curBDRBMoSJ7GLUujtDjycv2JXuIIqTrWNwVZr3tW4/YKBxUcD1SnBmffgoHDFlo9dTq03YAjg/sv2r/rcVb1xBwTJejQdOyVmL/QdLPLSWj3vgFWpA+jw1bV57sYyfawONZ5+hfemlZLClOjxZgD/IZ2/PmXlbStamj2uXbo2MrUiA4y/VhLCzvXZIeh5gs7WdxZXoanh0l9TAFISZtSr4GO60xL6GrbZ/oXf63WtDjZQ293eGr7xDMaMoF2qRycimZGQ3FX1+2N/peOM+p0PM0cgekP0rSqcLuRg3pq+dtF8guo9T/IhTIny25SmOSvmawc6pqXRO5WQI4hyfpNEmWLKTO9z/zU6nUGsDZMJMLFNnlGEwjpHZGO4ogQ1tyK8KWcQc",
"device_id": "RJPFIRDZYD",
"sender_key": "HlpwBwe9UsFhQ+WO7nJHI1CwjeXxPOOVCXE7zgzbCGg",
"session_id": "gAQxX0A0RM2b+up0ibRb68u0gEm8pDQgrtw9w9F9hCo"
},
"origin_server_ts": 1669207174410,
"sender": "@lgian:element.io",
"type": "m.room.encrypted",
"unsigned": {
"age": 1648859
},
"event_id": "$MK3coNBUiKdbiwJbX3BZnYQsRFJE2DE5K7LSy5RBfYg"
},
{
"content": {
"algorithm": "m.megolm.v1.aes-sha2",
"ciphertext": "AwgBEqABuFUjuZZaYZ0sd4tKqwZ3KBI1ULzLvINFHtPF053Bh8QqGDTx/Tgn6t5MFY/R1Oa7j1jNz6o5nH2kvj/30J5/w2e+R2ieZzjGhxzW1WsSu7DY+QTNipH4euorYp2R0BdawL+UHElQYZoYMg4ytVY+1fJRSF0aTPlXUjy4Wd1ZS9uzV9GtqMx4ImFtXmOeQSTUNEvllOEZxI4BAS5vuJt6HYrcLtuld7jiZlnvHHc8zuKLKqorMP06vuu4Bwx3OSy2zg5gf4ruXcAbP/jIv0mADpr2EK/Ez/eOmPlnKiDmCUUK6dfmNSnkAQ",
"device_id": "RJPFIRDZYD",
"sender_key": "HlpwBwe9UsFhQ+WO7nJHI1CwjeXxPOOVCXE7zgzbCGg",
"session_id": "gAQxX0A0RM2b+up0ibRb68u0gEm8pDQgrtw9w9F9hCo"
},
"origin_server_ts": 1669207397906,
"sender": "@lgian:element.io",
"type": "m.room.encrypted",
"unsigned": {
"age": 1425363
},
"event_id": "$XO_0WwJAi40qcMUdOFqB4xvdY1mQP7jjdiKPKIkdS9k"
},
{
"content": {
"avatar_url": "mxc://matrix.org/yqsMYtlqLZIlMOnqBRAXnkUm",
"displayname": "andybalaam",
"membership": "join"
},
"origin_server_ts": 1669208822225,
"sender": "@andybalaam:matrix.org",
"state_key": "@andybalaam:matrix.org",
"type": "m.room.member",
"unsigned": {
"age": 3,
"replaces_state": "$rWoSNtthPJ1WHmoO_qqvX-Q-Jmy9dTqKfKZFTw4idPg",
"prev_content": {
"avatar_url": "mxc://matrix.org/yqsMYtlqLZIlMOnqBRAXnkUm",
"displayname": "andybalaam",
"is_direct": true,
"membership": "invite"
},
"prev_sender": "@lgian:element.io"
},
"event_id": "$Wk2qUVwqsb8ive4l75sISoivifuXdm1l3iGEODpmm1s"
},
{
"content": {
"algorithm": "m.megolm.v1.aes-sha2",
"ciphertext": "AwgAEqABvDb35eiTzLREumzMNc9nezQNMwKRlh9r9nHaPg8F4EBrVh6YIp+lgEpzhuUHYqCDIVF0j9jRZ693IsUxemQnN4cJbwGo/1GAqFovLZNXV6Q7MYxukgtPiVnyFNsYnd8yGHG90TiGRO5Wl8BJQ+gzqRqKjt6Ums3c8iNeSDmmwYrYxA+LcRGVTaFiey+ixg49XQ+R8Y8msVFnNzF1e48F/ZKBNs+ToaA3ks0younKFNIK0UEV0/z/evDcBsSqUf9W/23u/9RodXEswHHhUS6eE44HC9iOi7m6LZBt0YFMWfteBsE9gxMSDw",
"device_id": "MKBDKTQJQJ",
"sender_key": "Ss6eaJGA82Uc9TQn1BbcLqpm6jtjGGwqMBadf/8Y+WY",
"session_id": "8RV7u4xrlnPO5yvqZpx/aqiHbq3+KTTYg193IpzzFPY"
},
"origin_server_ts": 1669208839345,
"sender": "@andybalaam:matrix.org",
"type": "m.room.encrypted",
"unsigned": {
"age": 4412625253
},
"event_id": "$dER5V1RCMxzAhHXQJoMjqyuoxpPtK2X6hCb9T8Jg2wU"
}
],
"prev_batch": "s3615474541_757284974_4801760_1841502201_1869275239_3913936_731927069_6351869257_0",
"limited": false
},
"state": {
"events": []
},
"account_data": {
"events": [
{
"type": "m.fully_read",
"content": {
"event_id": "$dER5V1RCMxzAhHXQJoMjqyuoxpPtK2X6hCb9T8Jg2wU"
}
}
]
},
"ephemeral": {
"events": [
{
"type": "m.receipt",
"content": {
"$XO_0WwJAi40qcMUdOFqB4xvdY1mQP7jjdiKPKIkdS9k": {
"m.read": {
"@andybalaam:matrix.org": {
"ts": 1669208824249
}
},
"m.read.private": {
"@andybalaam:matrix.org": {
"ts": 1669208824233
}
}
},
"$dER5V1RCMxzAhHXQJoMjqyuoxpPtK2X6hCb9T8Jg2wU": {
"m.read": {
"@lgian:element.io": {
"ts": 1669208845306
}
}
}
}
}
]
},
"unread_notifications": {
"notification_count": 1,
"highlight_count": 0
},
"summary": {
"m.joined_member_count": 2,
"m.invited_member_count": 0,
"m.heroes": [
"@lgian:element.io"
]
}
},
```

### Steps to reproduce

.

### Homeserver

matrix.org

### Synapse Version

{"server_version":"1.75.0rc1 (b=matrix-org-hotfixes,6a185c8b17)","python_version":"3.8.12"}

### Installation Method

I don't know

### Database

.

### Workers

Multiple workers

### Platform

.

### Configuration

.

### Relevant log output

```shell
.
```

### Anything else that would be useful to know?

.

The report provides a Matrix sync response for room !KCcFbqJHMQscdVsFTa:element.io and identifies unread_notifications, read receipts, and the fully-read event. Start by tracing the sync handling that calculates notification_count, then compare the latest sender and read state. Done means a room whose latest event was sent by the user does not report an unread notification; the issue names no source file or test.

Written by the indexing model from the issue text.