From 333f47a85abfe4ee7d801fe3e545dd5b4761bdbf Mon Sep 17 00:00:00 2001
From: Oleksandr Fediashov <olfedias@microsoft.com>
Date: Mon, 18 May 2026 12:17:51 +0200
Subject: [PATCH] perf: drop canOverrideNativeFocus runtime probe
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The `canOverrideNativeFocus` helper detected whether reassigning
`HTMLElement.prototype.focus` was respected by the browser. The check
was guarding against pre-IE9 behaviour where prototype overrides were
ignored.

Every browser keyborg currently supports honours the override, so the
`_canOverrideNativeFocus` flag is effectively always true after the
first call. Replace the conditional `details.isFocusedProgrammatically`
write with an unconditional one — semantically identical when the
override works, which is the only case we still ship for — and drop the
probe entirely.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 src/FocusEvent.mts | 34 +++-------------------------------
 1 file changed, 3 insertions(+), 31 deletions(-)

diff --git a/src/FocusEvent.mts b/src/FocusEvent.mts
index cacb6fafe..1cdba78e6 100644
--- a/src/FocusEvent.mts
+++ b/src/FocusEvent.mts
@@ -37,27 +37,6 @@ interface WindowWithKeyborgFocusEvent extends Window {
   __keyborgData?: KeyborgFocusEventData;
 }
 
-function canOverrideNativeFocus(win: Window): boolean {
-  const HTMLElement = (win as WindowWithKeyborgFocusEvent).HTMLElement;
-  const origFocus = HTMLElement.prototype.focus;
-
-  let isCustomFocusCalled = false;
-
-  HTMLElement.prototype.focus = function focus(): void {
-    isCustomFocusCalled = true;
-  };
-
-  const btn = win.document.createElement("button");
-
-  btn.focus();
-
-  HTMLElement.prototype.focus = origFocus;
-
-  return isCustomFocusCalled;
-}
-
-let _canOverrideNativeFocus = false;
-
 export interface KeyborgFocusInEventDetails {
   relatedTarget?: HTMLElement;
   isFocusedProgrammatically?: boolean;
@@ -97,11 +76,6 @@ export function setupFocusEvent(win: Window): void {
   const kwin = win as WindowWithKeyborgFocusEvent;
   const doc = kwin.document;
   const proto = kwin.HTMLElement.prototype;
-
-  if (!_canOverrideNativeFocus) {
-    _canOverrideNativeFocus = canOverrideNativeFocus(kwin);
-  }
-
   const origFocus = proto.focus;
 
   if ((origFocus as KeyborgFocus).__keyborgNativeFocus) {
@@ -258,12 +232,10 @@ export function setupFocusEvent(win: Window): void {
     // Tabster (and other users) can still use the legacy details field - keeping for backwards compat
     event.details = details;
 
-    if (_canOverrideNativeFocus || data[LAST_FOCUSED_PROGRAMMATICALLY]) {
-      details.isFocusedProgrammatically =
-        target === data[LAST_FOCUSED_PROGRAMMATICALLY]?.deref();
+    details.isFocusedProgrammatically =
+      target === data[LAST_FOCUSED_PROGRAMMATICALLY]?.deref();
 
-      data[LAST_FOCUSED_PROGRAMMATICALLY] = undefined;
-    }
+    data[LAST_FOCUSED_PROGRAMMATICALLY] = undefined;
 
     target.dispatchEvent(event);
   };