Don't run the editing shortcuts in a read-only composer

[FrayxRulez]

UnauthorizedAccessException — "Access is denied. (Exception from HRESULT: 0x80070005)" — reported by crash telemetry on 12.9.1.0.

   3  Windows::UI::Text::ITextRange__Impl::Dispatcher.ITextRange.SetText
   4  Telegram::Controls::FormattedTextBox.OnKeyDown
      Telegram\Controls\FormattedTextBox.cs:404
   5  Telegram::Controls::Chats::ChatTextBox.OnKeyDown
      Telegram\Controls\Chats\ChatTextBox.cs:313

Cause

ChatView makes the composer read-only whenever the user cannot post — chat.Permissions.CanSendBasicMessages is false and friends — while leaving it visible and focusable. FormattedTextBox.OnKeyDown never looked at IsReadOnly, so with send-by-Enter off, a plain Enter took the "insert a vertical tab instead of a hard paragraph" path and called Document.Selection.SetText on a document the app itself had write-protected. RichEdit returns E_ACCESSDENIED from ITextRange::SetText in that state, and the marshaller turns it into an exception.

Pressing Enter in any chat you can't post in is enough to reproduce it.

Every other branch of that switch — the emoji-aware Backspace/Delete, redo, the formatting toggles, Ctrl+K, Alt+X — mutates the document the same way and had the same hole.

Change

One early-out at the top of FormattedTextBox.OnKeyDown: when IsReadOnly, defer to the base implementation and skip the shortcuts, which is what the class already does at its other IsReadOnly guards. RichEdit ignores its own editing shortcuts on a read-only document, so nothing that used to work stops working.

Not built: no UWP/.NET Native toolchain here. CSharpSyntaxTree.ParseText on the edited file reports no diagnostics — that confirms it still parses, and nothing more.

🤖 Generated with Claude Code