The composer completes mentions and hashtags

A mention or hashtag being written at the end of the words is offered, once typing pauses, the accounts or tags
PrivaPub already knows (/api/v2/search, never resolving); a click writes it out. e2e: a town persona's handle,
begun, is offered and completed. CLAUDE.md also says that FEP-5624's moderated comments (PeerTube) show as pending
on a card, as GoToSocial's policies do.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Components/Composer.razor

@@ -1,3 +1,4 @@
+@using System.Text.RegularExpressions
 @inherits LocalizableComponentBase
 
 @* A new post or a reply, as the current persona. Through the Mastodon API it is plain text with a content warning, a
@@ -17,11 +18,28 @@
 		</DInput>
 	}
 	<div class="relative">
-		<DTextarea TValue="string" @bind-Value="form.Content" Immediate="true"
+		<DTextarea TValue="string" @bind-Value="form.Content" @bind-Value:after="Typed" Immediate="true"
 				   class="@(form.HasContentWarning ? "w-full rounded-b-[1.4rem] rounded-t-lg" : "w-full rounded-[1.4rem]")" maxlength="@ComposeForm.MaxContentLength" autocomplete="off"
 				   placeholder="@Placeholder" rows="3" data-compose />
 		<span class="absolute text-xs opacity-50 right-3 bottom-2">@ContentLength/@ComposeForm.MaxContentLength</span>
 	</div>
+	@if (suggestedAccounts.Count > 0 || suggestedTags.Count > 0)
+	{
+		<div class="flex flex-wrap gap-2" data-suggestions>
+			@foreach (var account in suggestedAccounts)
+			{
+				<DButton Size="DSize.Xs" class="max-w-full" OnClick="@(() => Complete("@" + account.Acct))" data-suggestion="@("@" + account.Acct)">
+					@* alt="": the handle is next to it. *@
+					<img alt="" class="h-4 w-4 flex-none rounded-full object-cover" src="@(account.Avatar ?? "imgs/icon-192.png")" />
+					<span class="truncate">@("@" + account.Acct)</span>
+				</DButton>
+			}
+			@foreach (var tag in suggestedTags)
+			{
+				<DButton Size="DSize.Xs" OnClick="@(() => Complete("#" + tag.Name))" data-suggestion="@("#" + tag.Name)">#@tag.Name</DButton>
+			}
+		</div>
+	}
 
 	@if (Quoting is not null)
 	{
@@ -215,6 +233,7 @@
 
 @code {
 	[Inject] StatusService StatusService { get; set; }
+	[Inject] AccountService AccountService { get; set; }
 	[Inject] ILoggingService Logger { get; set; }
 	[Inject] IJSRuntime JS { get; set; }
 
@@ -231,11 +250,51 @@
 
 	ComposeForm form = new();
 	List<string> fileErrors = [];
+	// what the words end with while a mention or a hashtag is being written: its sign and what follows it
+	static readonly Regex Typing = new(@"(?:^|\s)([@#])([\p{L}\p{N}_][\p{L}\p{N}_.\-]*(?:@[\p{L}\p{N}.\-]*)?)$", RegexOptions.Compiled);
+	List<Account> suggestedAccounts = [];
+	List<StatusTag> suggestedTags = [];
+	// the newest lookup: an older one's answer comes too late to show
+	int suggesting;
 	string error;
 	bool isPublishing;
 
 	int ContentLength => form.Content?.Length ?? 0;
 
+	// what is being written, once typing pauses, among the accounts and hashtags this server knows
+	async Task Typed()
+	{
+		var typed = Typing.Match(form.Content ?? "");
+		var lookup = ++suggesting;
+		if (!typed.Success || typed.Groups[2].Length < 2)
+		{
+			suggestedAccounts = [];
+			suggestedTags = [];
+			return;
+		}
+		await Task.Delay(300);
+		if (lookup != suggesting)
+			return;
+		var hashtags = typed.Groups[1].Value == "#";
+		var found = await AccountService.Suggest(typed.Groups[2].Value, hashtags);
+		if (lookup != suggesting)
+			return;
+		suggestedAccounts = hashtags ? [] : [.. found.Accounts.Take(5)];
+		suggestedTags = hashtags ? [.. found.Hashtags.Take(5)] : [];
+	}
+
+	// the mention or hashtag being written, written out
+	void Complete(string written)
+	{
+		var typed = Typing.Match(form.Content ?? "");
+		suggesting++;
+		suggestedAccounts = [];
+		suggestedTags = [];
+		if (!typed.Success)
+			return;
+		form.Content = form.Content[..typed.Groups[1].Index] + written + " ";
+	}
+
 	string Placeholder => Editing is not null ? Localizer["Edit your post"]
 		: Quoting is not null ? Localizer["Say something about it"]
 		: InReplyTo is null ? Localizer["What's happening?"]