From 1d67b993dc9072f36a7742fc3d284b7bd935a84d Mon Sep 17 00:00:00 2001
From: Daneel <daneel@sukany.cz>
Date: Sun, 1 Mar 2026 04:56:16 +0100
Subject: [PATCH 10/11] perf: use BUF_CHARS_MODIFF for AX text cache validity
 in ensureTextCache

The cache validity check in -[EmacsAccessibilityBuffer ensureTextCache]
used BUF_MODIFF, which is bumped by every text-property change ---
including face applications by font-lock on each redisplay cycle.
Since the AX text value contains only characters (no face or property
data), property-only changes do not affect it.  Rebuilding the full
buffer text on each font-lock pass is O(buffer-size) per redisplay,
causing progressive slowdown proportional to how far the cursor is
from the beginning of the file.

Switch to BUF_CHARS_MODIFF, which is bumped only when characters are
actually inserted or deleted.  This matches the semantic of 'did the
text change' and is the approach used by WebKit and NSTextView.

BUF_OVERLAY_MODIFF is intentionally not tracked here (unchanged):
overlay content is handled by separate announcements in
postAccessibilityNotificationsForFrame, and including it in this
check would prevent those announcements from firing.
---
 src/nsterm.m | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/src/nsterm.m b/src/nsterm.m
index 12c451b..e3f9466 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -8148,16 +8148,25 @@ - (void)ensureTextCache
   if (!b)
     return;
 
-  ptrdiff_t modiff = BUF_MODIFF (b);
-  ptrdiff_t pt = BUF_PT (b);
-  NSUInteger textLen = cachedText ? [cachedText length] : 0;
-  /* Cache validity: track BUF_MODIFF and buffer narrowing.
+  /* Use BUF_CHARS_MODIFF, not BUF_MODIFF, for cache validity.
+     BUF_MODIFF is bumped by every text-property change, including
+     font-lock face applications on every redisplay.  AX text contains
+     only characters, not face data, so property-only changes do not
+     affect the cached value.  Rebuilding the full buffer text on
+     each font-lock pass is O(buffer-size) per redisplay --- this
+     causes progressive slowdown when scrolling through large files.
+     BUF_CHARS_MODIFF is bumped only on actual character insertions
+     and deletions, matching the semantic of "did the text change".
+     This is the pattern used by WebKit and NSTextView.
      Do NOT track BUF_OVERLAY_MODIFF here --- overlay text is not
      included in the cached AX text (it is handled separately via
-     explicit announcements).  Including overlay_modiff would
-     silently update cachedOverlayModiff and prevent the
-     notification dispatch from detecting overlay changes.  */
-  if (cachedText && cachedTextModiff == modiff
+     explicit announcements in postAccessibilityNotificationsForFrame).
+     Including overlay_modiff would silently update cachedOverlayModiff
+     and prevent the notification dispatch from detecting changes.  */
+  ptrdiff_t chars_modiff = BUF_CHARS_MODIFF (b);
+  ptrdiff_t pt = BUF_PT (b);
+  NSUInteger textLen = cachedText ? [cachedText length] : 0;
+  if (cachedText && cachedTextModiff == chars_modiff
       && cachedTextStart == BUF_BEGV (b)
       && pt >= cachedTextStart
       && (textLen == 0
@@ -8173,7 +8182,7 @@ included in the cached AX text (it is handled separately via
     {
       [cachedText release];
       cachedText = [text retain];
-      cachedTextModiff = modiff;
+      cachedTextModiff = chars_modiff;
       cachedTextStart = start;
 
       if (visibleRuns)
-- 
2.43.0