feat(ui): add scroll-to-bottom button and improve scroll navigation

components/frontend/src/components/session/MessagesTab.tsx

@@ -1,10 +1,11 @@
 "use client";
 
 import React, { useState, useRef, useEffect, useMemo, useLayoutEffect, useCallback } from "react";
-import { MessageSquare, ChevronUp } from "lucide-react";
+import { MessageSquare, ChevronUp, ChevronDown } from "lucide-react";
 import { StreamMessage } from "@/components/ui/stream-message";
 import { LoadingDots } from "@/components/ui/message";
 import { Button } from "@/components/ui/button";
+import { Tooltip, TooltipContent, TooltipProvider, TooltipTrigger } from "@/components/ui/tooltip";
 import { ChatInputBox } from "@/components/chat/ChatInputBox";
 import { QueuedMessageBubble } from "@/components/chat/QueuedMessageBubble";
 import { useCurrentUser } from "@/services/queries/use-auth";
@@ -99,6 +100,7 @@ const MessagesTab: React.FC<MessagesTabProps> = ({ session, streamMessages, chat
   const checkIfAtBottom = () => {
     const container = messagesContainerRef.current;
     if (!container) return true;
+    if (container.scrollHeight <= container.clientHeight) return true;
     const threshold = 50;
     return container.scrollHeight - container.scrollTop - container.clientHeight < threshold;
   };
@@ -287,17 +289,42 @@ const MessagesTab: React.FC<MessagesTabProps> = ({ session, streamMessages, chat
         )}
       </div>
 
-      {showScrollToTop && (
-        <Button
-          variant="outline"
-          size="icon-sm"
-          onClick={scrollToTop}
-          aria-label="Scroll to top"
-          className="absolute bottom-3 right-5 z-10 rounded-full shadow-md transition-opacity duration-200"
-        >
-          <ChevronUp className="h-4 w-4" />
-        </Button>
-      )}
+      <TooltipProvider>
+        <div className="absolute bottom-3 right-5 z-10 flex flex-col gap-1">
+          <div className={`transition-all duration-200 ${showScrollToTop ? "opacity-100 scale-100" : "opacity-0 scale-75 pointer-events-none"}`}>
+            <Tooltip open={showScrollToTop ? undefined : false}>
+              <TooltipTrigger asChild>
+                <Button
+                  variant="outline"
+                  size="icon-sm"
+                  onClick={scrollToTop}
+                  aria-label="Scroll to top"
+                  className="rounded-full shadow-md cursor-pointer"
+                >
+                  <ChevronUp className="h-4 w-4" />
+                </Button>
+              </TooltipTrigger>
+              <TooltipContent side="left">Scroll to top</TooltipContent>
+            </Tooltip>
+          </div>
+          <div className={`transition-all duration-200 ${!isAtBottom ? "opacity-100 scale-100" : "opacity-0 scale-75 pointer-events-none"}`}>
+            <Tooltip open={isAtBottom ? false : undefined}>
+              <TooltipTrigger asChild>
+                <Button
+                  variant="outline"
+                  size="icon-sm"
+                  onClick={() => messagesContainerRef.current?.scrollTo({ top: messagesContainerRef.current.scrollHeight, behavior: "smooth" })}
+                  aria-label="Scroll to bottom"
+                  className="rounded-full shadow-md cursor-pointer"
+                >
+                  <ChevronDown className="h-4 w-4" />
+                </Button>
+              </TooltipTrigger>
+              <TooltipContent side="left">Scroll to bottom</TooltipContent>
+            </Tooltip>
+          </div>
+        </div>
+      </TooltipProvider>
       </div>
 
       <ChatInputBox

specs/frontend/sessions/messages/scroll-navigation.spec.md

@@ -0,0 +1,115 @@
+# Session Message Scroll Navigation
+
+## Purpose
+
+The session message view SHALL provide contextual scroll navigation buttons that allow users to quickly navigate to the top or bottom of the message list without manual scrolling.
+
+## Requirements
+
+### Requirement: Scroll-to-Top Button Visibility
+
+The message view SHALL display a scroll-to-top button when the user has scrolled past a threshold distance from the top of the message container.
+
+#### Scenario: Button appears after scrolling down
+
+- GIVEN a message list with content taller than the viewport
+- WHEN the user scrolls past a threshold distance from the top
+- THEN a scroll-to-top button becomes visible
+
+#### Scenario: Button hidden near the top
+
+- GIVEN the scroll-to-top button is visible
+- WHEN the user scrolls back within the threshold distance of the top
+- THEN the button becomes hidden
+
+### Requirement: Scroll-to-Bottom Button Visibility
+
+The message view SHALL display a scroll-to-bottom button when the user is not at the bottom of the message list.
+
+#### Scenario: Button appears when scrolled up
+
+- GIVEN a message list with content taller than the viewport
+- WHEN the user scrolls up so the bottom of the content is not visible
+- THEN a scroll-to-bottom button becomes visible
+
+#### Scenario: Button hidden at bottom
+
+- GIVEN the scroll-to-bottom button is visible
+- WHEN the user scrolls to near the bottom of the content
+- THEN the button becomes hidden
+
+### Requirement: No Buttons on Short Content
+
+Neither scroll button SHALL appear when the message content does not overflow the container.
+
+#### Scenario: Few messages
+
+- GIVEN a message list shorter than the viewport height
+- WHEN the view renders
+- THEN neither scroll button is visible
+
+### Requirement: Smooth Animated Transitions
+
+Both buttons SHALL animate in and out smoothly rather than appearing or disappearing abruptly. Hidden buttons SHALL NOT trigger hover effects or tooltips.
+
+#### Scenario: Button fades in
+
+- GIVEN a scroll button is hidden
+- WHEN its visibility condition becomes true
+- THEN the button transitions to visible smoothly
+
+#### Scenario: Button fades out
+
+- GIVEN a scroll button is visible
+- WHEN its visibility condition becomes false
+- THEN the button transitions to hidden smoothly
+- AND the button does not receive pointer events while hidden
+- AND the button's tooltip does not appear while hidden
+
+### Requirement: Smooth User-Initiated Scrolling
+
+Both buttons SHALL scroll smoothly when activated by user click.
+
+#### Scenario: Scroll to top
+
+- GIVEN the scroll-to-top button is visible
+- WHEN the user clicks it
+- THEN the message container scrolls smoothly to the top
+
+#### Scenario: Scroll to bottom
+
+- GIVEN the scroll-to-bottom button is visible
+- WHEN the user clicks it
+- THEN the message container scrolls smoothly to the bottom
+
+### Requirement: Instant Auto-Scroll During Streaming
+
+Auto-scroll that keeps the viewport pinned to new messages during streaming SHALL remain instant and not use smooth scrolling.
+
+#### Scenario: New message during streaming
+
+- GIVEN the user is at the bottom of the message list
+- WHEN a new streaming message or token arrives
+- THEN the container scrolls instantly to show the new content
+- AND the scroll is not animated
+
+### Requirement: Button Tooltips
+
+Both buttons SHALL display a tooltip describing their action.
+
+#### Scenario: Tooltip content
+
+- GIVEN either scroll button is visible
+- WHEN the user hovers over the button
+- THEN a tooltip appears with the text "Scroll to top" or "Scroll to bottom" respectively
+
+### Requirement: Button Layout
+
+Both buttons SHALL be positioned in the bottom-right corner of the message container, stacked vertically with the scroll-to-top button above the scroll-to-bottom button.
+
+#### Scenario: Both buttons visible
+
+- GIVEN the user has scrolled past the top threshold and is not at the bottom
+- WHEN both buttons are visible
+- THEN they appear stacked vertically in the bottom-right corner of the message area
+- AND the scroll-to-top button is above the scroll-to-bottom button