gnachman · chall37 · Feb 11, 2026 · Feb 11, 2026 · Feb 11, 2026
diff --git a/ModernTests/VT100ScreenTests.swift b/ModernTests/VT100ScreenTests.swift
diff --git a/sources/VT100ScreenMutableState+Private.h b/sources/VT100ScreenMutableState+Private.h
@@ -57,6 +57,9 @@ iTermTriggerScopeProvider> {
     iTermKittyImageController *_kittyImageController;
     // When YES, terminal input is collected into printBuffer for ANSI print commands.
     BOOL _collectInputForPrinting;
+    // Precomputed fast-path eligibility. Recomputed at each state-mutation point.
+    // Access on mutation queue only
+    BOOL _fastPathEligible;
 }
 
 @property (atomic) BOOL hadCommand;
@@ -67,6 +70,9 @@ iTermTriggerScopeProvider> {
 @property (class, atomic, readwrite) BOOL performingJoinedBlock;
 @property (nonatomic) BOOL allowNextReport;
 
+- (BOOL)_computeFastPathEligible;
+- (void)_recomputeFastPathEligible;
+
 - (iTermEventuallyConsistentIntervalTree *)mutableIntervalTree;
 - (iTermEventuallyConsistentIntervalTree *)mutableSavedIntervalTree;
 - (iTermColorMap *)mutableColorMap;

diff --git a/sources/VT100ScreenMutableState+Resizing.m b/sources/VT100ScreenMutableState+Resizing.m
@@ -1371,7 +1371,7 @@ - (void)reallySetSize:(VT100GridSize)newSize
                    newTop:newTop
                  delegate:delegate];
     [altScreenLineBuffer endResizing];
-    self.commandStartCoord = [self startCoordOfCurrentCommand];
+    [self setCommandStartCoordWithoutSideEffects:[self startCoordOfCurrentCommand]];
     [self sanityCheckIntervalsFrom:oldSize note:@"post-hoc"];
     DLog(@"After:\n%@", [self.currentGrid compactLineDumpWithContinuationMarks]);
     DLog(@"Cursor at %d,%d", self.currentGrid.cursorX, self.currentGrid.cursorY);

diff --git a/sources/VT100ScreenMutableState+TerminalDelegate.m b/sources/VT100ScreenMutableState+TerminalDelegate.m
@@ -232,17 +232,19 @@ - (VT100ScreenMutableStateAppendInfo)appendInfoForTokens:(NSArray<VT100Token *>
 }
 
 - (void)terminalAppendMixedAsciiGang:(NSArray<VT100Token *> *)tokens {
-    VT100Terminal *terminal = self.terminal;
-    BOOL canTakeFastPath = (!_collectInputForPrinting &&
-                            ![self shouldEvaluateTriggers] &&
-                            ![self shouldConvertCharactersToGraphicsCharacterSetInTerminal:terminal] &&
-                            !self.config.publishing &&
-                            self.commandStartCoord.x == -1 &&
+    // Self-healing recompute: catch staleness from lazy expiry of expectations
+    // (deadlines pass without notification). Only runs on the slow path so
+    // the fast path stays a single ivar read.
+    if (!_fastPathEligible) {
+        [self _recomputeFastPathEligible];
+    }
+#if DEBUG
+    ITAssertWithMessage(_fastPathEligible == [self _computeFastPathEligible],
+                        @"Stale _fastPathEligible: cached=%d computed=%d",
+                        _fastPathEligible, [self _computeFastPathEligible]);
+#endif
+    BOOL canTakeFastPath = (_fastPathEligible &&
                             tokens.count > 0 &&
-                            self.wraparoundMode &&
-                            !self.ansi &&
-                            !self.insert &&
-                            self.currentGrid == self.primaryGrid &&
                             self.currentGrid.canTakeFastPath);
     if (!canTakeFastPath) {
         const int numLines = [self appendGangSlowPath:tokens];
@@ -840,6 +842,7 @@ - (void)terminalPrintBuffer {
     DLog(@"string is: %@", string);
     self.printBuffer = nil;
     _collectInputForPrinting = NO;
+    [self _recomputeFastPathEligible];
     // Pause so that attributes like colors don't change until printing (which is async) can begin.
     [self addPausedSideEffect:^(id<VT100ScreenDelegate> delegate, iTermTokenExecutorUnpauser *unpauser) {
         DLog(@"begin side-effect");
@@ -855,6 +858,7 @@ - (void)terminalPrintScreen {
     // Print out the whole screen
     self.printBuffer = nil;
     _collectInputForPrinting = NO;
+    [self _recomputeFastPathEligible];
 
     // Pause so we print the current state and not future updates.
     [self addPausedSideEffect:^(id<VT100ScreenDelegate> delegate, iTermTokenExecutorUnpauser *unpauser) {
@@ -874,6 +878,7 @@ - (void)terminalBeginRedirectingToPrintBuffer {
     // allocate a string for the stuff to be printed
     self.printBuffer = [[NSMutableString alloc] init];
     _collectInputForPrinting = YES;
+    [self _recomputeFastPathEligible];
 }
 
 - (void)terminalSetWindowTitle:(NSString *)title {
@@ -2106,16 +2111,23 @@ - (void)terminalSetShellIntegrationVersion:(NSString *)version {
 - (void)terminalWraparoundModeDidChangeTo:(BOOL)newValue {
     DLog(@"begin %@", @(newValue));
     self.wraparoundMode = newValue;
+    [self _recomputeFastPathEligible];
 }
 
 - (void)terminalTypeDidChange {
     DLog(@"begin");
     self.ansi = [self.terminal isAnsi];
+    [self _recomputeFastPathEligible];
 }
 
 - (void)terminalInsertModeDidChangeTo:(BOOL)newValue {
     DLog(@"begin %@", @(newValue));
     self.insert = newValue;
+    [self _recomputeFastPathEligible];
+}
+
+- (void)terminalActiveCharsetDidChangeTo:(int)charset {
+    [self _recomputeFastPathEligible];
 }
 
 - (int)terminalChecksumInRectangle:(VT100GridRect)rect {

diff --git a/sources/VT100ScreenMutableState.m b/sources/VT100ScreenMutableState.m
@@ -132,6 +132,7 @@ - (instancetype)initWithSideEffectPerformer:(id<VT100ScreenSideEffectPerforming>
         _promptStateMachine.delegate = self;
         _kittyImageController = [[iTermKittyImageController alloc] init];
         _kittyImageController.delegate = self;
+        [self _recomputeFastPathEligible];
     }
     return self;
 }
@@ -205,6 +206,7 @@ - (void)setTerminalEnabled:(BOOL)enabled {
         self.ansi = self.terminal.isAnsi;
         self.wraparoundMode = self.terminal.wraparoundMode;
         self.insert = self.terminal.insertMode;
+        [self _recomputeFastPathEligible];
         _commandRangeChangeJoiner = [iTermIdempotentOperationJoiner joinerWithScheduler:_tokenExecutor];
         _terminal.delegate = self;
         _tokenExecutor.delegate = self;
@@ -476,6 +478,7 @@ - (void)setConfig:(VT100MutableScreenConfiguration *)config {
         [_promptStateMachine revealOrDismissComposerAgain];
     }
     _tokenExecutor.isBackgroundSession = !config.sessionIsVisible;
+    [self _recomputeFastPathEligible];
 }
 
 - (void)movePromptUnderComposerIfNeeded {
@@ -981,6 +984,26 @@ - (BOOL)shouldConvertCharactersToGraphicsCharacterSetInTerminal:(VT100Terminal *
     return [self.charsetUsesLineDrawingMode containsObject:@(terminal.charset)];
 }
 
+// Mutation queue only
+- (BOOL)_computeFastPathEligible {
+    return (!_collectInputForPrinting &&
+            !_triggerEvaluator.haveTriggersOrExpectations &&
+            !self.config.loggingEnabled &&
+            _postTriggerActions.count == 0 &&
+            ![self shouldConvertCharactersToGraphicsCharacterSetInTerminal:self.terminal] &&
+            !self.config.publishing &&
+            self.commandStartCoord.x == -1 &&
+            self.wraparoundMode &&
+            !self.ansi &&
+            !self.insert &&
+            self.currentGrid == self.primaryGrid);
+}
+
+// Mutation queue only
+- (void)_recomputeFastPathEligible {
+    _fastPathEligible = [self _computeFastPathEligible];
+}
+
 - (void)reverseIndex {
     if (self.currentGrid.cursorY == self.currentGrid.topMargin) {
         if (self.cursorOutsideLeftRightMargin) {
@@ -1945,6 +1968,7 @@ - (void)showAltBuffer {
     [self hideOnScreenNotesAndTruncateSpanners];
     self.currentGrid = self.altGrid;
     self.currentGrid.cursor = self.primaryGrid.cursor;
+    [self _recomputeFastPathEligible];
 
     [self swapOnscreenIntervalTreeObjects];
     [self reloadMarkCache];
@@ -1965,6 +1989,7 @@ - (void)showPrimaryBuffer {
     [self.temporaryDoubleBuffer reset];
     [self hideOnScreenNotesAndTruncateSpanners];
     self.currentGrid = self.primaryGrid;
+    [self _recomputeFastPathEligible];
     [self invalidateCommandStartCoordWithoutSideEffects];
     [self swapOnscreenIntervalTreeObjects];
     [self reloadMarkCache];
@@ -2214,6 +2239,7 @@ - (void)setCharacterSet:(int)charset usesLineDrawingMode:(BOOL)lineDrawingMode {
     } else {
         [self.charsetUsesLineDrawingMode removeObject:@(charset)];
     }
+    [self _recomputeFastPathEligible];
 }
 
 #pragma mark - Tabs
@@ -3320,6 +3346,7 @@ - (void)setHost:(NSString *)host
 
 - (void)setCoordinateOfCommandStart:(VT100GridAbsCoord)coord {
     self.commandStartCoord = coord;
+    [self _recomputeFastPathEligible];
     [self didUpdatePromptLocation];
     [self commandRangeDidChange];
 }
@@ -3406,6 +3433,7 @@ - (void)removePromptMarksBelowLine:(int)line {
 
 - (void)setCommandStartCoordWithoutSideEffects:(VT100GridAbsCoord)coord {
     self.commandStartCoord = coord;
+    [self _recomputeFastPathEligible];
 }
 
 - (void)invalidateCommandStartCoordWithoutSideEffects {
@@ -4590,6 +4618,7 @@ - (void)injectData:(NSData *)data {
 
 - (void)evaluateTriggers:(void (^ NS_NOESCAPE)(PTYTriggerEvaluator *triggerEvaluator))block {
     block(_triggerEvaluator);
+    [self _recomputeFastPathEligible];
     if (_tokenExecutor.isExecutingToken) {
         return;
     }
@@ -4602,6 +4631,7 @@ - (void)executePostTriggerActions {
     }
     NSArray<void (^)(void)> *actions = [_postTriggerActions copy];
     [_postTriggerActions removeAllObjects];
+    [self _recomputeFastPathEligible];
     for (void (^block)(void) in actions) {
         block();
     }
@@ -4860,6 +4890,7 @@ - (void)updateExpectFrom:(iTermExpect *)source {
     DLog(@"Update expect %@", source);
     _triggerEvaluator.expect = [source copy];
     DLog(@"Mutation thread expectations are now %@", _triggerEvaluator.expect);
+    [self _recomputeFastPathEligible];
 }
 
 - (void)performLightweightBlockWithJoinedThreads:(void (^ NS_NOESCAPE)(VT100ScreenMutableState *mutableState))block {
@@ -5137,6 +5168,7 @@ - (void)restoreFromDictionary:(NSDictionary *)dictionary
             }
             DLog(@"------------ end -----------");
         }
+        [self _recomputeFastPathEligible];
     }
 }
 
@@ -5341,6 +5373,7 @@ - (void)restoreFromSavedState:(NSDictionary *)terminalState {
     if (path) {
         [self setPathFromURL:path];
     }
+    [self _recomputeFastPathEligible];
 }
 
 #pragma mark - iTermMarkDelegate
@@ -6065,6 +6098,7 @@ - (void)triggerSession:(Trigger *)trigger didDetectPromptAtAbsLine:(long long)li
     [_postTriggerActions addObject:[^{
         [weakSelf handleTriggerDetectedPromptAt:range];
     } copy]];
+    [self _recomputeFastPathEligible];
     if (_tokenExecutor.isExecutingToken) {
         self.terminal.wantsDidExecuteCallback = YES;
     }

diff --git a/sources/VT100Terminal.m b/sources/VT100Terminal.m
@@ -547,8 +547,11 @@ - (void)setSendReceiveMode:(BOOL)sendReceiveMode {
 }
 
 - (void)setCharset:(int)charset {
-    self.dirty = YES;
-    _charset = charset;
+    if (_charset != charset) {
+        self.dirty = YES;
+        _charset = charset;
+        [_delegate terminalActiveCharsetDidChangeTo:charset];
+    }
 }
 
 - (void)setCursorMode:(BOOL)cursorMode {

diff --git a/sources/VT100TerminalDelegate.h b/sources/VT100TerminalDelegate.h
@@ -435,6 +435,7 @@ typedef NS_ENUM(NSUInteger, VT100TerminalProtectedMode) {
 - (void)terminalWraparoundModeDidChangeTo:(BOOL)newValue;
 - (void)terminalTypeDidChange;
 - (void)terminalInsertModeDidChangeTo:(BOOL)newValue;
+- (void)terminalActiveCharsetDidChangeTo:(int)charset;
 
 - (NSString *)terminalProfileName;