diff --git a/apps/backend/internal/agent/lifecycle/manager.go b/apps/backend/internal/agent/lifecycle/manager.go
index 400e0f4c6b..00c125cd96 100644
--- a/apps/backend/internal/agent/lifecycle/manager.go
+++ b/apps/backend/internal/agent/lifecycle/manager.go
@@ -1578,16 +1578,22 @@ func (m *Manager) flushMessageBuffer(execution *AgentExecution) string {
 		}
 	}
 
-	// If we have remaining content and an active streaming message, append it
+	// If we have remaining message content, publish it
 	trimmedMessage := strings.TrimSpace(agentMessage)
-	if trimmedMessage != "" && currentMsgID != "" {
-		// Publish final append to the streaming message
-		m.publishStreamingMessageFinal(execution, currentMsgID, trimmedMessage)
+	if trimmedMessage != "" {
+		if currentMsgID != "" {
+			// Publish final append to the streaming message
+			m.publishStreamingMessageFinal(execution, currentMsgID, trimmedMessage)
+		} else {
+			// No streaming message exists yet - create one with all the content
+			// This happens when message content has no newlines (never triggered streaming)
+			m.publishStreamingMessage(execution, trimmedMessage)
+		}
 		// Return empty since we've already handled it via streaming
 		return ""
 	}
 
-	return trimmedMessage
+	return ""
 }
 
 // publishStreamingMessageFinal publishes the final chunk of a streaming message.
diff --git a/apps/web/components/diff/diff-viewer.tsx b/apps/web/components/diff/diff-viewer.tsx
index c8bbfb9fef..fcc72d19cc 100644
--- a/apps/web/components/diff/diff-viewer.tsx
+++ b/apps/web/components/diff/diff-viewer.tsx
@@ -452,7 +452,9 @@ export const DiffViewer = memo(function DiffViewer({
       lineDiffType: 'word',
       overflow: wordWrap ? 'wrap' : 'scroll',
       unsafeCSS: `
-        * {
+        /* Make container backgrounds transparent, but NOT diff line backgrounds
+         * Only target the root container and specific UI elements, not the diff content itself */
+        [data-diffs-root] {
           background: transparent !important;
         }