Where TDLib vectors are mutated

Every place the app writes into a list that came from TDLib, collected while deciding whether to expose vector<x> as an immutable Vector<T> rather than IList<T>. Useful on its own: a list reaching the app from an update is shared with the cached object it came from, so a write into one is visible to every other reader of that object.

How this was collected

No single method sees all of it, so three were used and unioned.

Compile against an immutable type. The generator was flipped to emit Vector<T> and the app built. The compiler reports a mutation only where the concrete type reaches the call, so this sees 32 sites and is blind to anything that has already widened to IList<T>.
Match the schema against mutating calls. All 238 vector<...> field names were read out of td_api.tl and matched against Add/Insert/Remove/RemoveAt/Clear/Sort/AddRange and indexer writes. 259 raw hits, most of them noise: Items, Blocks, Values and Attributes are schema field names and XAML or settings properties, so flyout.Items.Add(...) matches. 44 survive filtering.
Make the widening visible. IList<T> and the non-generic IList were removed from Vector<T> while IReadOnlyList<T> was kept, and the app rebuilt. Every point where a vector escapes into a slot that could mutate it then fails to compile, while foreach and LINQ stay quiet. 212 escapes, resolved to their sink below.

1 and 2 overlap on 25 sites; 7 are compiler-only and 19 grep-only, which is the measure of how partial either is alone.

What to do with each group

A is the real list: the app writes directly into a vector it does not own.
B and C are one step removed - the vector is handed to something that mutates it, or stored in a field that is mutated later. Same effect, one indirection away.
D is the bulk and needs no thought: 140 escapes into a parameter that only reads. Tightening the declaration from IList<T> to the concrete type is enough, and is what buys TG1001 the visibility the whole exercise is for.

Two shapes are mixed together in A and worth separating before touching anything. ClientService mutating a cached object under lock (value) to apply an update is deliberate and correct today; immutability turns each into a rebuild, which costs an allocation on the update path but hands readers a consistent snapshot instead of a torn one. Everything in Controls\ and most of ViewModels\ is the other shape: app code editing a list it was merely handed, where the existing defensive copies nearby (ChatCell.xaml.cs:1520, ClientEx.cs MergeEntities) show the invariant was already understood and just not enforced.

Caveats

Groups 2 and 3 rest on textual heuristics - a name match for the first, a one-hop sink resolution for the second - so this is a floor, not a proof. A vector passed through two levels of IList<T> parameter, or stored in a collection of lists, is not caught. 27 escapes resolved to neither a call nor an assignment and were left out; most are return x.SomeVector; from an IList<T>-returning member, which propagates the question to that member's callers rather than answering it.

A. Direct mutation of a TDLib vector property

Common\RichHtml.cs:279 — items[items.Count - 1].Blocks.Add(block);
Common\TLNavigationService.cs:259 — features.Features.Remove(appIcons);
Common\TLNavigationService.cs:265 — features.Limits.Remove(archivedChats);
Common\TLNavigationService.cs:268 — features.Limits.Add(new PremiumLimit(new PremiumLimitTypeConnectedAccounts(), 3, 4));
Controls\Cells\ChatCell.xaml.cs:1521 — message.Entities.Add(new TextEntity(match.Index, match.Length, new TextEntityTypeSpoiler
Controls\Chats\ChatTranslateBar.xaml.cs:173 — markdown.Entities.Add(new TextEntity(index, 2, new TextEntityTypeCustomEmoji(51972528272
Controls\Chats\ChatTranslateBar.xaml.cs:179 — link.Entities.Add(new TextEntity(link.Entities[0].Offset, link.Entities[0].Length, new T
Controls\Messages\EmojiMenuFlyout.xaml.cs:690 — _message.UnreadReactions.Add(unread);
Controls\Messages\EmojiMenuFlyout.xaml.cs:692 — _message.UnreadReactions.Remove(unread);
Controls\Messages\MessageEffectMenuFlyout.xaml.cs:423 — _message.UnreadReactions.Add(unread);
Controls\Messages\MessageEffectMenuFlyout.xaml.cs:425 — _message.UnreadReactions.Remove(unread);
Controls\Messages\MessageServiceText.cs:1408 — content.Entities.Add(new TextEntity(index, 2, new TextEntityTypeCustomEmoji(forumTopicEd
Controls\Messages\MessageServiceText.cs:2551 — if (source.Entities.IsReadOnly)
Controls\Messages\MessageServiceText.cs:2637 — source.Entities.Add(new TextEntity(index, name.Length, id ?? new TextEntityTypeBold()));
Controls\Messages\MessageServiceText.cs:2693 — if (source.Entities.IsReadOnly)
Controls\Messages\MessageServiceText.cs:2738 — source.Entities.AddRange(entities);
Controls\Messages\ReactionsMenuFlyout.xaml.cs:1088 — _message.UnreadReactions.Add(unread);
Controls\Messages\ReactionsMenuFlyout.xaml.cs:1090 — _message.UnreadReactions.Remove(unread);
Services\ClientService.cs:3551 — value.ChatLists.Add(updateChatAddedToList.ChatList);
Services\ClientService.cs:3568 — value.ChatLists.Remove(chatList);
Td\ClientEx.cs:92 — merge.Entities.Add(entity);
ViewModels\Business\BusinessBotsViewModel.cs:582 — recipients.ChatIds.Add(chat.ChatId);
ViewModels\Business\BusinessBotsViewModel.cs:590 — recipients.ExcludedChatIds.Add(chat.ChatId);
ViewModels\Business\BusinessRecipientsViewModelBase.cs:158 — recipients.ChatIds.Add(chat.ChatId);
ViewModels\ChatListViewModel.cs:524 — folder.ExcludedChatIds.Remove(data.Chat.Id);
ViewModels\ChatListViewModel.cs:525 — folder.IncludedChatIds.Add(data.Chat.Id);
ViewModels\ChatListViewModel.cs:544 — folder.IncludedChatIds.Remove(data.Chat.Id);
ViewModels\ChatListViewModel.cs:561 — folder.IncludedChatIds.Remove(data.Chat.Id);
ViewModels\ChatListViewModel.cs:562 — folder.ExcludedChatIds.Add(data.Chat.Id);
ViewModels\Chats\ChatStoriesViewModel.cs:249 — story?.AlbumIds.Add(album.Id);
ViewModels\Chats\ChatStoriesViewModel.cs:278 — story.AlbumIds.Add(album.Id);
ViewModels\DialogViewModel.cs:1455 — foundChatMessages.Messages.Insert(i, album.MessagesValue[j]);
ViewModels\DialogViewModel.cs:1610 — messages.MessagesValue.Insert(index + 1, new Message(0, target.SenderId, null, target.Ch
ViewModels\DialogViewModel.cs:888 — messages.MessagesValue.RemoveAt(messages.MessagesValue.Count - 1);
ViewModels\Drawers\EmojiDrawerViewModel.cs:313 — defaultStickers.StickersValue.Insert(0, new Sticker(0, 0, 0, 0, string.Empty, null, null
ViewModels\Folders\FolderViewModel.cs:444 — folder.PinnedChatIds.Add(item);
ViewModels\Folders\FolderViewModel.cs:473 — folder.IncludedChatIds.Add(chat.ChatId);
ViewModels\Folders\FolderViewModel.cs:496 — folder.ExcludedChatIds.Add(chat.ChatId);
ViewModels\Folders\FolderViewModel.cs:95 — folder.IncludedChatIds.Add(createArgs.IncludeChatId);
ViewModels\Premium\PromoViewModel.cs:78 — features.Features.Remove(appIcons);
ViewModels\Premium\PromoViewModel.cs:84 — features.Limits.Remove(archivedChats);
ViewModels\Premium\PromoViewModel.cs:87 — features.Limits.Add(new PremiumLimit(new PremiumLimitTypeConnectedAccounts(), 3, 4));
ViewModels\Profile\ProfileGiftsTabViewModel.cs:211 — gift?.CollectionIds.Add(collection.Id);
ViewModels\Profile\ProfileGiftsTabViewModel.cs:240 — gift.CollectionIds.Add(collection.Id);
ViewModels\Profile\ProfileGiftsTabViewModel.cs:255 — param.gift.CollectionIds.Remove(param.collection.Id);
ViewModels\Profile\ProfileGiftsTabViewModel.cs:268 — param.gift.CollectionIds.Add(param.collection.Id);
ViewModels\Settings\SettingsStorageViewModel.cs:284 — chat.ByFileType.Remove(fileType);
ViewModels\Settings\SettingsStorageViewModel.cs:294 — result.ByFileType.Add(already);
ViewModels\Settings\SettingsStorageViewModel.cs:303 — value.ByChat.Remove(chat);
Views\MainPage.xaml.cs:3287 — response.IncludedChatIds.Remove(chat.Id);
Views\Popups\MemberTagInfoPopup.xaml.cs:96 — markdown.Entities.Add(new TextEntity(index, 3, new TextEntityTypeMention()));

B. Passed into a helper whose IList<T> parameter it mutates

Common\PageBlockHelper.cs:1166 → TryAppendInputBlocks(blocks) — if (TryAppendInputBlocks(source.Blocks, text, entities))
Common\PageBlockHelper.cs:1190 → Flatten(entities) — Flatten(GetRichText(message?.Blocks), text, entities);
Common\TextBlockHelper.cs:207 → GetRuns(entities) — var runs = TextStyleRun.GetRuns(text, entities);
Common\TextStyleRun.cs:126 → GetRuns(entities) — return GetRuns(formatted.Text, formatted.Entities);
Common\TextStyleRun.cs:404 → StyledText(entities) — return new StyledText(text.Text, text.Entities, GetParagraphs(text.Text, text.Entities));
Common\TextStyleRun.cs:421 → GetText(entities) — return GetText(PageBlockHelper.GetRichText(message.Blocks));
ViewModels\DialogViewModel.cs:894 → AddHeaderAsync(messages) — await AddHeaderAsync(messages.MessagesValue, fromMessage?.Get());
ViewModels\Drawers\AnimationDrawerViewModel.cs:67 → Merge(destination) — BeginOnUIThread(() => Merge(SavedItems, animation.AnimationsValue));
ViewModels\Drawers\AnimationDrawerViewModel.cs:78 → Merge(destination) — BeginOnUIThread(() => Merge(SavedItems, animation.AnimationsValue));
ViewModels\Drawers\StickerDrawerViewModel.cs:91 → Merge(destination) — BeginOnUIThread(() => Merge(_favoriteSet.Stickers, favorite.StickersValue));
ViewModels\Drawers\StickerDrawerViewModel.cs:121 → Merge(destination) — BeginOnUIThread(() => Merge(_recentSet.Stickers, recent.StickersValue));

C. Stored into a field or property that is mutated elsewhere

Services\ClientService.cs:4257 → Messages — value.Messages = updateQuickReplyShortcutMessages.Messages;
Services\ClientService.cs:4263 → Messages — Messages = updateQuickReplyShortcutMessages.Messages
ViewModels\DialogViewModel.Handle.cs:1112 → UnreadReactions — message.UnreadReactions = update.UnreadReactions;
ViewModels\Dialogs\DialogPendingTextMessage.cs:343 → _pending — _pending = messageRich.Message.Blocks;
ViewModels\MessageViewModel.cs:193 → UnreadReactions — UnreadReactions = message.UnreadReactions;
ViewModels\MessageViewModel.cs:497 → UnreadReactions — UnreadReactions = message.UnreadReactions;
ViewModels\Stories\StoryViewModel.cs:100 → AlbumIds — AlbumIds = story.AlbumIds;
Views\InstantPage.xaml.cs:137 → Blocks — ViewModel.Blocks = instantView.Blocks;