feat(conversation): provide the session header corner slot

packages/client/ui-conversation/src/client/apply.ts

@@ -279,6 +279,7 @@ export function apply(ctx: Context, config: Config = Config({})): void {
       'conversation.session.header.lineage': { kind: 'single', scope: 'session' },
       'conversation.session.header.actions': { kind: 'list', scope: 'session' },
       'conversation.session.header.utilities': { kind: 'list', scope: 'session' },
+      'conversation.session.header.corner': { kind: 'single', scope: 'session' },
     },
     store: conversationStore,
     inject: (sessionId: SessionId, actions: BoundActions<typeof conversationStore>): ConversationSessionHeaderInjected => ({

packages/client/ui-conversation/src/client/contract/slots.ts

@@ -139,6 +139,18 @@ declare module '@deepseek-ai/dsh-client-ui-slots' {
       scope: 'session'
       owner: ConversationHeaderActionOwnerProps
     }
+    /**
+     * The header's far-right corner, past the utilities' edge and into the
+     * header's own padding, for one control that must keep its place whether or
+     * not it currently shows anything. The corner reserves its width while an
+     * occupant is registered, so the utilities beside it never move; an
+     * occupant with nothing to show renders a same-size placeholder.
+     */
+    'conversation.session.header.corner': {
+      kind: 'single'
+      scope: 'session'
+      owner: ConversationHeaderCornerOwnerProps
+    }
     /** Registered Conversation target Views, rendered one at a time. */
     'conversation.view': { kind: 'list'; scope: 'session'; owner: ConvViewOwnerProps }
     /** Selector-routed replacements for the current Session's resident composer. */
@@ -209,6 +221,12 @@ export interface ConversationHeaderActionOwnerProps {
   children?: never
 }
 
+/** The header corner's occupant derives its state from standard Session props. */
+export interface ConversationHeaderCornerOwnerProps {
+  /** Marker field: the occupant receives no owner-specific values. */
+  children?: never
+}
+
 /** Plain breadcrumb data handed to the optional lineage renderer. */
 export interface ConversationHeaderLineageOwnerProps {
   /** Session represented by this breadcrumb title. */
@@ -375,6 +393,7 @@ export type ConversationSessionHeaderSlotProps =
     'conversation.session.header.lineage'
     | 'conversation.session.header.actions'
     | 'conversation.session.header.utilities'
+    | 'conversation.session.header.corner'
   >
   & PropsStore<ConversationStore>
   & InjectFace<ConversationSessionHeaderInjected>

packages/client/ui-conversation/src/client/index.ts

@@ -51,7 +51,7 @@ export type {
   ComposerAttachment, ComposerAttachmentsOwnerProps, ComposerAttachmentsProps,
   ComposerFileAttachment, ComposerImageAttachment, DraftFileUpload, DraftFileUploads,
   ComposerBarInjected, ComposerBarOwnerProps, ComposerBarProps, ComposerChainProps,
-  ConversationHeaderActionOwnerProps, ConversationHeaderLineageOwnerProps,
+  ConversationHeaderActionOwnerProps, ConversationHeaderCornerOwnerProps, ConversationHeaderLineageOwnerProps,
   ConversationInjected, ConversationSessionHeaderInjected, ConversationSessionHeaderSlotProps,
   ConversationSessionInjected, ConversationSessionSlotProps, ConversationSlotProps,
   ConversationStore, ConvViewOwnerProps, ConvViewProps, EmptyWorkspaceOwnerProps,

packages/client/ui-conversation/src/client/skeleton/ConversationRoot.module.css

@@ -146,6 +146,22 @@
   display: none;
 }
 
+/* The far-right corner seat reaches 16px into the header's 28px right padding,
+   so its control sits past the utilities' edge; it is laid out only while an
+   occupant is registered, and the occupant keeps its width while hidden, so the
+   utilities never move because of it. */
+.headerCorner {
+  display: flex;
+  flex: none;
+  align-items: center;
+  margin-left: 12px;
+  margin-right: -16px;
+}
+
+.headerCorner:empty {
+  display: none;
+}
+
 /* figma Tab_Group 34:11441: 35px strip, gap 36, pad-left 8, tabs bottom-aligned. */
 .tabs {
   position: relative;

packages/client/ui-conversation/src/client/skeleton/ConversationSession.tsx

@@ -133,6 +133,9 @@ export function ConversationSessionHeader({
             <div className={css.headerUtilities}>
               {renderSlot('conversation.session.header.utilities', {})}
             </div>
+            <div className={css.headerCorner} data-conversation-header-corner="">
+              {renderSlot('conversation.session.header.corner', {})}
+            </div>
           </div>
           {tabs.length > 1 && (
             <div className={css.tabs} role="tablist">