fix(site): fix action bar hidden after null-returning assistant messages (#24566)

closes CODAGT-121

When an invisible assistant message (e.g. a `provider_executed`
tool-result with no text content) appeared between a visible assistant
message and the next user message, the visible assistant's action bar
was incorrectly hidden. The chain logic computed `isLastInChain` from
the raw `parsedMessages` array, which includes entries that
`ChatMessageItem` returns `null` for.

Extract a shared `isTimelineMessageVisible` helper that encodes the
three null-return conditions (`provider_executed` tool-result-only,
all-provider-executed parts, metadata-only) in one place. Use it both to
guard the early return in `ChatMessageItem` and to skip invisible
entries when computing `isLastInChain`, so chain boundaries are based on
the next *rendered* message.

site/src/pages/AgentsPage/components/ChatConversation/ConversationTimeline.stories.tsx

@@ -1607,3 +1607,65 @@ export const NoRenderableContentFallbackSpacing: Story = {
 		).toBeInTheDocument();
 	},
 };
+
+/**
+ * Regression: action bar must appear on the last *visible* assistant
+ * message even when invisible assistant messages (provider-executed
+ * tool-result-only) follow it before the next user turn.
+ */
+export const AssistantActionBarAfterHiddenMessages: Story = {
+	args: {
+		...defaultArgs,
+		parsedMessages: buildMessages([
+			{
+				...baseMessage,
+				id: 1,
+				role: "user",
+				content: [{ type: "text", text: "Help me refactor" }],
+			},
+			{
+				...baseMessage,
+				id: 2,
+				role: "assistant",
+				content: [
+					{ type: "text", text: "Here is the **refactored** version." },
+				],
+			},
+			{
+				...baseMessage,
+				id: 3,
+				role: "assistant",
+				content: [
+					{
+						type: "tool-result",
+						tool_call_id: "provider-tool-1",
+						result: { output: "done" },
+						provider_executed: true,
+					},
+				],
+			},
+			{
+				...baseMessage,
+				id: 4,
+				role: "user",
+				content: [{ type: "text", text: "Thanks!" }],
+			},
+		]),
+	},
+	play: async ({ canvasElement }) => {
+		const canvas = within(canvasElement);
+		// Force the hover-reveal action bars visible using stable test IDs.
+		for (const el of canvasElement.querySelectorAll(
+			'[data-testid="message-actions"]',
+		)) {
+			if (el instanceof HTMLElement) {
+				el.style.opacity = "1";
+			}
+		}
+		// 2 user messages + 1 visible assistant = 3 action bars.
+		// The invisible provider-executed tool-result message (id=3)
+		// must not prevent the assistant (id=2) from showing its bar.
+		const actions = canvas.getAllByTestId("message-actions");
+		expect(actions).toHaveLength(3);
+	},
+};

site/src/pages/AgentsPage/components/ChatConversation/ConversationTimeline.tsx

@@ -821,6 +821,28 @@ const StickyUserMessage = memo<{
 	},
 );
 
+function computeLastInChainFlags(
+	parsedMessages: readonly ParsedMessageEntry[],
+): boolean[] {
+	const flags = new Array<boolean>(parsedMessages.length).fill(false);
+	let nextVisibleIsUser = true; // no next visible => treat as chain end
+	for (let i = parsedMessages.length - 1; i >= 0; i--) {
+		const entry = parsedMessages[i];
+		const { shouldHide } = deriveMessageDisplayState({
+			message: entry.message,
+			parsed: entry.parsed,
+			hideActions: false,
+		});
+		if (entry.message.role !== "user") {
+			flags[i] = nextVisibleIsUser;
+		}
+		if (!shouldHide) {
+			nextVisibleIsUser = entry.message.role === "user";
+		}
+	}
+	return flags;
+}
+
 interface ConversationTimelineProps {
 	parsedMessages: readonly ParsedMessageEntry[];
 	subagentTitles: Map<string, string>;
@@ -854,6 +876,8 @@ export const ConversationTimeline = memo<ConversationTimelineProps>(
 		mcpServers,
 		showDesktopPreviews,
 	}) => {
+		const lastInChainFlags = computeLastInChainFlags(parsedMessages);
+
 		if (parsedMessages.length === 0) {
 			return null;
 		}
@@ -929,10 +953,10 @@ export const ConversationTimeline = memo<ConversationTimelineProps>(
 								/>
 							);
 						}
-						// Hide actions on assistant messages that are not
-						// the last in a consecutive assistant chain.
-						const next = parsedMessages[msgIdx + 1];
-						const isLastInChain = !next || next.message.role === "user";
+						// Hide actions on assistant messages that are not the
+						// last in a consecutive assistant chain. Flags are
+						// precomputed in a single reverse pass above.
+						const isLastInChain = lastInChainFlags[msgIdx];
 						return (
 							<ChatMessageItem
 								key={message.id}