Triage
Confirmed. With Send shortcut set to ⌘ + Enter always, pressing ⌘+Enter on a new thread starts it in the background instead of sending normally. With the Enter setting, ⌘+Enter on a new thread still starts it in the background, and that should stay.
composerSubmissionIntentForEnter in apps/web/src/composer-logic.ts already treats ⌘/Ctrl+Enter as the send key when sendShortcut is mod-enter (and when it's mod-enter-multiline and the draft has a newline). But the last line ignores that and still returns "background" whenever the modifier is held on a draft:
apps/web/src/composer-logic.ts (line 44):
| 1 | return input.modifierKey && input.isDraftThread ? "background" : "foreground"; |
That's what opens a fresh draft and shows the "Started in background" toast. After the first message the thread is no longer a draft, so the same shortcut sends normally, which matches what you saw in the video.
In this mode there's currently no keyboard shortcut for a normal first send. Enter adds a newline, and so does ⌘+Shift+Enter. The Send button still works as expected. The same problem shows up with ⌘ + Enter for multiline prompts once the draft has a newline in it. A single-line draft in that mode is fine, because Enter sends and ⌘+Enter can keep meaning background.
This has been the behavior since the send-shortcut setting landed, and the line is unchanged on current main. The keybindings doc also still says mod+Enter always starts a new thread in the background, which is only true when Enter itself sends.
The fix is to return "background" only when the modifier isn't needed to send, and to add tests for mod-enter and multiline mod-enter-multiline on a draft. Background start would then need a different shortcut in those modes. ⌘+Shift+Enter is free on a draft, since it's only used as the follow-up override while a turn is running.
Thanks for the clear repro and the recording.