fix(site): add padding below thinking-only assistant messages (#24140)

closes CODAGT-122 

Add a spacer div that renders only when an assistant message lacks the
action bar, matching the height the action bar would provide.

> 🤖 Generated by Coder Agents

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

@@ -896,3 +896,46 @@ export const MultiAssistantTurnCopyButton: Story = {
 		expect(actions).toHaveLength(2);
 	},
 };
+
+/**
+ * Regression: thinking-only assistant messages must have consistent
+ * bottom spacing before the next user bubble. A spacer div fills the
+ * gap that would normally come from the invisible action bar.
+ */
+export const ThinkingOnlyAssistantSpacing: Story = {
+	args: {
+		...defaultArgs,
+		parsedMessages: buildMessages([
+			{
+				...baseMessage,
+				id: 1,
+				role: "user",
+				content: [{ type: "text", text: "Explain this code" }],
+			},
+			{
+				...baseMessage,
+				id: 2,
+				role: "assistant",
+				content: [
+					{
+						type: "reasoning",
+						text: "Let me think about this step by step. The user wants me to explain the code they shared.",
+					},
+				],
+			},
+			{
+				...baseMessage,
+				id: 3,
+				role: "user",
+				content: [{ type: "text", text: "Any progress?" }],
+			},
+		]),
+	},
+	play: async ({ canvasElement }) => {
+		const canvas = within(canvasElement);
+		// The thinking-only assistant message has no action bar, but
+		// it should still have visible text and a spacer element.
+		expect(canvas.getByText("Explain this code")).toBeInTheDocument();
+		expect(canvas.getByText("Any progress?")).toBeInTheDocument();
+	},
+};

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

@@ -669,6 +669,13 @@ const ChatMessageItem = memo<{
 							)}
 						</div>
 					)}
+				{/* Spacer for assistant messages without an action bar
+				   (e.g. thinking-only) so they have consistent bottom
+				   padding before the next user bubble. */}
+				{!hideActions &&
+					!isUser &&
+					!hasCopyableContent &&
+					Boolean(parsed.reasoning) && <div className="min-h-6" />}
 				{previewImage && (
 					<ImageLightbox
 						src={previewImage}