docs: add LINE client settings guide

README.md

@@ -336,3 +336,7 @@ automatically.
 
 For details on how the bridge handles different Letter Sealing
 combinations, see [readme/LETTER_SEALING.md](readme/LETTER_SEALING.md).
+
+For LINE app settings that change how a bridged message looks or plays (GIF
+autoplay, auto-download, photo quality), see
+[readme/LINE_CLIENT_SETTINGS.md](readme/LINE_CLIENT_SETTINGS.md).

readme/LINE_CLIENT_SETTINGS.md

@@ -0,0 +1,64 @@
+# LINE client settings that change what a message looks like
+
+LINE's own apps have settings that decide how a received message is shown or
+played. Beeper has no equivalent. Read this before you test a feature that
+touches media, previews, or notifications.
+
+Verified on 2026-10-25 with LINE iOS and LINE Chrome Extension 3.7.2.
+
+## Why it matters
+
+The same message can look different on two LINE clients. A GIF can autoplay on
+one and wait for a tap on another, with no bridge bug involved. If you do not
+know which setting is on, you can read a normal result as a bug, or miss a real
+one.
+
+## Settings by area
+
+"Unknown" means nobody has checked it yet. Do not assume it matches iOS.
+
+| Setting | What it does | iOS | Android | Desktop | Chrome extension | Beeper equivalent | What to test |
+|---|---|---|---|---|---|---|---|
+| Photo quality (Settings → Photos and videos) | Standard or High for photos you send | Yes | Unknown | Unknown | `Photo resolution` in Settings → Chats (default `Original`; photos over 20 MB can't be sent in their original resolution) | None | Photos at both values. Does not change GIFs: the original file is sent either way |
+| Auto-download images | Downloads received images on its own | Yes | Unknown | Unknown | Unknown | None | With it off, media sent from another device waits for a tap before it downloads |
+| Autoplay GIFs | Plays received GIFs without a tap | Yes | Unknown | Unknown | `Animated GIFs`: "Auto-play animated GIFs in chats" | None | GIF on and off. See the sender and receiver rules below |
+| Autoplay videos | Plays received videos without a tap | Yes | Unknown | Unknown | Unknown | None | Check that a GIF does not arrive as a video, because this setting would then control it |
+| Send Live Photos | Sends iOS Live Photos as such | Yes | Unknown | Unknown | Unknown | None | Only for Live Photo work. Not about GIFs |
+| File previews | Preview of attached files | Unknown | Unknown | Unknown | Settings → Chats | None | Files of different types |
+| URL previews | Preview of links | Unknown | Unknown | Unknown | Settings → Chats | None | A link with and without a preview |
+| Font size, Send key, Chat folders | Layout and input | Unknown | Unknown | Unknown | Settings → Chats | None | Rarely needed |
+| Block messages from non-friends | Drops messages from people who are not friends | Unknown | Unknown | Unknown | Settings → Chats | None | Messages from a non-friend, if your feature involves one |
+| Notifications: pop-up, sound, show preview, rules | What an incoming message does | Unknown | Unknown | Unknown | Settings → Notifications | Beeper has its own | Only if your feature changes notification text |
+| Turn off mobile notifications when logged in to the Chrome version | Mutes the phone while the extension is logged in | Unknown | Unknown | Unknown | Settings → Notifications | None | Remember the bridge acts as the Chrome extension, so this can mute the phone |
+
+## Sender versus receiver
+
+- Playback and auto-download settings apply to what the device **receives**.
+  A test that only looks at your own sent message can miss them. Photo quality
+  and Send Live Photos apply on the sending device.
+- LINE iOS autoplays a GIF only if it was sent from that phone. A GIF sent from
+  another device of the same account, whether the Chrome extension or the
+  bridge, needs a tap there. So a bridge-sent GIF that does not autoplay on your
+  own phone is not a bug.
+- The Chrome extension autoplays all GIFs.
+- With Auto-download images off, media from another device waits for a tap
+  before it downloads.
+- The real receiver test is a second account. One tester cannot check it.
+
+## Testing without a second account
+
+- Use the Chrome extension as a separate view of your own chat. It receives
+  messages with no cached file. LINE allows only one Chrome extension session,
+  so logging into the extension logs the bridge out. Quit Chrome and reconnect
+  the bridge in Beeper when you finish.
+- Record each LINE setting before you start and put it back afterwards.
+- Do not use a LINE Official Account bot that relays messages through the
+  Messaging API as a receiver for media tests. It re-sends with its own
+  metadata and flattens GIFs to one frame.
+
+## Always capture what the Chrome extension does first
+
+The bridge copies the Chrome extension, so the extension is the reference. Before
+you write code for a feature, capture what the extension sends and receives, and
+match that. The tools and the workflow are in
+[beeper-line-skills](https://github.com/clins1994/beeper-line-skills).