Trace window focus, blur and visibility in pen input

From the development repository at 0304046.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/js/pen_input.js

@@ -178,7 +178,8 @@ export function attachPenInput(surface, target) {
   /** @param {PointerEvent} event @returns {string}  The press, for the trace. */
   function describe(event) {
     const where = event.target === surface ? '' : ` on ${describeTarget(event.target)}`;
-    return `${event.pointerType} ${event.pointerId} buttons ${event.buttons} pressure ${event.pressure.toFixed(2)}${where}`;
+    const focus = document.hasFocus() ? 'in front' : 'behind';
+    return `${event.pointerType} ${event.pointerId} buttons ${event.buttons} pressure ${event.pressure.toFixed(2)} (${focus})${where}`;
   }
 
   surface.addEventListener('pointerdown', (event) => {
@@ -314,6 +315,20 @@ export function attachPenInput(surface, target) {
   // The side button of a pen asks for a context menu.
   surface.addEventListener('contextmenu', (event) => event.preventDefault());
 
+  // The window's focus, blur and the tab's visibility, traced into the
+  // Session log but acting on nothing yet (2026-10-11): Andy's pages
+  // sometimes gained a lone dot when he clicked the page to bring the
+  // browser to the front from the terminal, with the Pen chosen.  A
+  // press that only raised the window should draw nothing, but whether
+  // the browser reports the focus before that press or after it, and
+  // whether the document is focused when the press arrives, must be
+  // learned on his machine before the rule is written (a window focus
+  // is faked by the test harness on every tab switch, so it cannot be
+  // learned here).  The press trace already carries "in front"/"behind".
+  window.addEventListener('focus', () => trace('window: focus'));
+  window.addEventListener('blur', () => trace('window: blur'));
+  document.addEventListener('visibilitychange', () => trace(`visibility: ${document.visibilityState}`));
+
   // Safari on iOS: cancelling pointerdown leaves the touch's own
   // behaviour (selection, the magnifier, the callout) to WebKit; only a
   // cancelled touchstart stops it.  The text field is a child of the

tests/test_end_to_end.py

@@ -781,7 +781,7 @@ class EndToEndTest(unittest.TestCase):
         # and a release of the pen, after the browser's name.
         shown = firefox.run("return document.getElementById('event_log').textContent;")
         self.assertRegex(shown, r"^\d\d:\d\d:\d\d\.\d\d\d Browser: Mozilla")
-        self.assertRegex(shown, r"\n\S+ press pen \d+ buttons 1 pressure \d\.\d\d, pen\n")
+        self.assertRegex(shown, r"\n\S+ press pen \d+ buttons 1 pressure \d\.\d\d \((?:in front|behind)\), pen\n")
         self.assertRegex(shown, r"\n\S+ pointerup pen \d+, \d+ points")
         firefox.run("document.getElementById('session_close').click();")
         self.assertFalse(firefox.run("return document.getElementById('session_dialog').open;"))