fix: preserve scroll position in session viewer when scrolled up#28
Merged
wesbillman merged 2 commits intomainfrom Feb 11, 2026
Merged
fix: preserve scroll position in session viewer when scrolled up#28wesbillman merged 2 commits intomainfrom
wesbillman merged 2 commits intomainfrom
Conversation
Track whether the user is near the bottom of the messages container using an onscroll handler. During polling, only auto-scroll to bottom if the user was already near the bottom (within 80px). Intentional scrolls (initial load, user sends a message) still force scroll to bottom unconditionally. - Add isNearBottom state and handleScroll listener on modal-content - Add scrollToBottomIfNear() that gates on isNearBottom - Replace scrollToBottom() with scrollToBottomIfNear() in poll() - Keep unconditional scrollToBottom() in loadSession() and sendMessage()
Replace the fragile isNearBottom heuristic (< 80px from bottom) with directional intent tracking using lastScrollTop comparison. This properly detects user-initiated upward scrolling across all input methods (mouse wheel, keyboard, touch, scrollbar drag). - Track userScrolledUp flag set when scrollTop decreases (upward scroll) - Clear flag when user scrolls back to exact bottom or on explicit scrollToBottom() calls (initial load, sending a message) - Remove magic 80px threshold that could misfire on short containers or miss intent on long ones
wesbillman
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Feb 11, 2026
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Nice! I'm always amazed at how hard this is for every chat app. 😂
loganj
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Feb 26, 2026
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Problem
When viewing a live session in the session modal, polling for new messages would always auto-scroll to the bottom — even if the user had intentionally scrolled up to read earlier content. This made it impossible to review previous messages during an active session.
Solution
Commit 1: Basic scroll preservation
Track whether the user is near the bottom of the messages container. During polling, only auto-scroll if the user was already at the bottom. Intentional scrolls (initial load, user sends a message) still force scroll to bottom unconditionally.
Commit 2: Replace pixel threshold with intent tracking
The initial implementation used a fragile 80px pixel threshold to determine "near bottom." This was replaced with directional intent tracking — comparing
scrollTopagainstlastScrollTopto detect when the user has scrolled upward.isNearBottom— true when within 80px of bottomuserScrolledUp— true when user scrolls upwardKey behavior:
userScrolledUp = truewhenscrollTopdecreases (user scrolled up)scrollToBottom()callsChanges
src/lib/SessionModal.svelte— scroll tracking logic in the messages container