interactive: keep viewport anchored when streaming below

scrollOffset is measured from the bottom of the chat buffer, so when
the agent appends new lines while the user has scrolled up to read
history, the visible window slides down through the buffer and the
content the user was reading drifts off the top.

Track the previous chat line count and column width across redraws.
While the user is in free-scroll (scrollOffset > 0) and the terminal
hasn't been resized, bump scrollOffset by the chat-length delta so
the visible content stays pinned. Compensation is skipped on resize
(line counts aren't comparable across reflows) and when following
the tail (scrollOffset == 0), where new content should keep pushing
the viewport as before.

commit: ec79818512897853ae730e8b8afbca40f8d44c55
author: patriceckhart · 5mo ago · 2026-04-27 07:42 UTC
parent: 5905729
signature: unsigned
refs: tag: v0.1.24

1 file · +35 -0

M internal/agent/modes/interactive.go +35 -0


@@ -192,6 +192,17 @@ type Interactive struct {
192	192		scrollOffset int // rows from the bottom; 0 = pinned to latest
193	193		prevScrollOffset int // last value redraw snapped against; tracks intent
194	194
	195	+	// prevChatLen and prevChatCols track the chat buffer's size at the
	196	+	// last redraw so that when content grows below the user's viewport
	197	+	// while they're scrolled up reading history, we can bump
	198	+	// scrollOffset by exactly the growth and keep the visible content
	199	+	// pinned. Without this, every streamed line shifts the visible
	200	+	// window down through the buffer (because scrollOffset is measured
	201	+	// from the bottom) and the user's reading position drifts upward
	202	+	// and off the top.
	203	+	prevChatLen int
	204	+	prevChatCols int
	205	+
195	206		// Messages typed while a turn is in flight. Each is delivered as
196	207		// its own follow-up turn once the current one finishes. Rendered
197	208		// above the status bar as "sliding in: ..." chips.

@@ -815,6 +826,30 @@ func (i *Interactive) redraw() {
815	826		chatRows = 1
816	827		}
817	828
	829	+	// Auto-follow guard: when the user has scrolled up (scrollOffset
	830	+	// > 0) and the agent appends new content below the viewport while
	831	+	// they're reading, compensate so the visible content stays
	832	+	// anchored. scrollOffset is measured from the bottom of `chat`,
	833	+	// so without compensation a growing buffer pushes the window
	834	+	// downward through the content and the lines the user was
	835	+	// reading scroll off the top.
	836	+	//
	837	+	// Skip compensation when the terminal width changed (a resize
	838	+	// reflows the whole buffer and the line-count delta no longer
	839	+	// corresponds to appended content) and when scrollOffset is 0
	840	+	// (the user is following the tail and wants new content to push
	841	+	// the view down as usual).
	842	+	if i.scrollOffset > 0 && i.prevChatCols == cols && i.prevChatLen > 0 {
	843	+	if delta := len(chat) - i.prevChatLen; delta != 0 {
	844	+	i.scrollOffset += delta
	845	+	if i.scrollOffset < 0 {
	846	+	i.scrollOffset = 0
	847	+	}
	848	+	}
	849	+	}
	850	+	i.prevChatLen = len(chat)
	851	+	i.prevChatCols = cols
	852	+
818	853		// Apply scroll offset to the chat slice.
819	854		maxOffset := len(chat) - chatRows
820	855		if maxOffset < 0 {


@@ -192,6 +192,17 @@ type Interactive struct {
192	scrollOffset int // rows from the bottom; 0 = pinned to latest	192		scrollOffset int // rows from the bottom; 0 = pinned to latest
193	prevScrollOffset int // last value redraw snapped against; tracks intent	193		prevScrollOffset int // last value redraw snapped against; tracks intent
194		194
		195	+	// prevChatLen and prevChatCols track the chat buffer's size at the
		196	+	// last redraw so that when content grows below the user's viewport
		197	+	// while they're scrolled up reading history, we can bump
		198	+	// scrollOffset by exactly the growth and keep the visible content
		199	+	// pinned. Without this, every streamed line shifts the visible
		200	+	// window down through the buffer (because scrollOffset is measured
		201	+	// from the bottom) and the user's reading position drifts upward
		202	+	// and off the top.
		203	+	prevChatLen int
		204	+	prevChatCols int
		205	+
195	// Messages typed while a turn is in flight. Each is delivered as	206		// Messages typed while a turn is in flight. Each is delivered as
196	// its own follow-up turn once the current one finishes. Rendered	207		// its own follow-up turn once the current one finishes. Rendered
197	// above the status bar as "sliding in: ..." chips.	208		// above the status bar as "sliding in: ..." chips.

@@ -815,6 +826,30 @@ func (i *Interactive) redraw() {
815	chatRows = 1	826		chatRows = 1
816	}	827		}
817		828
		829	+	// Auto-follow guard: when the user has scrolled up (scrollOffset
		830	+	// > 0) and the agent appends new content below the viewport while
		831	+	// they're reading, compensate so the visible content stays
		832	+	// anchored. scrollOffset is measured from the bottom of `chat`,
		833	+	// so without compensation a growing buffer pushes the window
		834	+	// downward through the content and the lines the user was
		835	+	// reading scroll off the top.
		836	+	//
		837	+	// Skip compensation when the terminal width changed (a resize
		838	+	// reflows the whole buffer and the line-count delta no longer
		839	+	// corresponds to appended content) and when scrollOffset is 0
		840	+	// (the user is following the tail and wants new content to push
		841	+	// the view down as usual).
		842	+	if i.scrollOffset > 0 && i.prevChatCols == cols && i.prevChatLen > 0 {
		843	+	if delta := len(chat) - i.prevChatLen; delta != 0 {
		844	+	i.scrollOffset += delta
		845	+	if i.scrollOffset < 0 {
		846	+	i.scrollOffset = 0
		847	+	}
		848	+	}
		849	+	}
		850	+	i.prevChatLen = len(chat)
		851	+	i.prevChatCols = cols
		852	+
818	// Apply scroll offset to the chat slice.	853		// Apply scroll offset to the chat slice.
819	maxOffset := len(chat) - chatRows	854		maxOffset := len(chat) - chatRows
820	if maxOffset < 0 {	855		if maxOffset < 0 {