Revert "Stop mapping stale-tail view lines to the wrong buffer line"

This reverts the freshViewLineCount stale-tail guard (§8), restoring the
demonstrate-the-bug test state. The upcoming off-screen render rebuilds
the displayed buffer wholesale on swap and lets refreshViewLinesIfNeeded
truncate, so the stale tail never forms — a cleaner fix than tracking a
fresh-count. Removing the guard on its own keeps that change focused; the
bug it guarded against is re-fixed by the truncation in the next commit.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/gocui/view.go

@@ -100,16 +100,6 @@ type View struct {
 	// true and viewLines to nil
 	viewLines []viewLine
 
-	// Number of leading entries in viewLines that correspond to the current
-	// buffer (v.buf.lines), as built by the most recent refreshViewLinesIfNeeded.
-	// refreshViewLinesIfNeeded overwrites viewLines in place without truncating,
-	// so when a re-render is shorter than the previous one the tail keeps stale
-	// entries from that previous render (drawn deliberately, to avoid flicker
-	// while the new content loads). Those entries don't correspond to the
-	// current buffer, so readers that map a view line to a buffer line must
-	// ignore them; this is the count of the entries they may trust.
-	freshViewLineCount int
-
 	// writeMutex protects locks the write process
 	writeMutex sync.Mutex
 
@@ -240,7 +230,6 @@ type pos struct {
 func (v *View) clearViewLines() {
 	v.tainted = true
 	v.viewLines = nil
-	v.freshViewLineCount = 0
 	v.clearHover()
 }
 
@@ -1513,7 +1502,6 @@ func (v *View) refreshViewLinesIfNeeded() {
 	}
 
 	v.firstDirtyLine = len(lines)
-	v.freshViewLineCount = lineIdx
 	v.tainted = false
 }
 
@@ -1754,15 +1742,19 @@ func (v *View) BufferLineForViewLine(y int) (int, bool) {
 func (v *View) bufferLineForViewLine(y int) (int, bool) {
 	v.refreshViewLinesIfNeeded()
 
-	// Bound on freshViewLineCount rather than len(v.viewLines): the entries past
-	// it are a stale tail retained for flicker-avoidance (see freshViewLineCount)
-	// and don't correspond to the current buffer. Within the fresh range every
-	// entry's linesY was just built from v.buf.lines, so it is guaranteed in range.
-	if y < 0 || y >= v.freshViewLineCount {
+	if y < 0 || y >= len(v.viewLines) {
 		return 0, false
 	}
 
-	return v.viewLines[y].linesY, true
+	// refreshViewLinesIfNeeded overwrites viewLines in place without truncating,
+	// so while a shorter re-render is loading, the tail of viewLines can still
+	// hold stale entries pointing past the (shrunk) v.buf.lines. Guard against that.
+	linesY := v.viewLines[y].linesY
+	if linesY >= len(v.buf.lines) {
+		return 0, false
+	}
+
+	return linesY, true
 }
 
 // indexFunc allows to split lines by words taking into account spaces

pkg/gocui/view_test.go

@@ -230,7 +230,10 @@ func TestBufferLineForViewLineStaleTail(t *testing.T) {
 	// buffer, so the mapping must fail. (On the buggy code it instead maps to
 	// buffer line 1, the stale entry's lingering index.)
 	_, ok = v.BufferLineForViewLine(4)
+	/* EXPECTED:
 	assert.False(t, ok)
+	ACTUAL: */
+	assert.True(t, ok)
 }
 
 func TestContainsColoredText(t *testing.T) {