A press that only brings the window to the front draws nothing

From the development repository at 62e0967.

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

client/js/pen_input.js

@@ -16,6 +16,31 @@ const ERASER_BUTTON = 32;
  */
 const NOMINAL_PRESSURE = 0.4;
 
+/**
+ * A press this soon after the window gained focus is taken to be the
+ * click that brought the window to the front, and draws nothing (Andy,
+ * 2026-10-11): clicking the page to return from the terminal, with the
+ * Pen chosen, left a stray dot.  Measured on Hollis (Firefox, Linux):
+ * the window's focus event fires about 4 ms before such a press, the
+ * document already reporting focus, so neither document.hasFocus() nor
+ * the pointer's movement (a pen hovers either way) can tell it from an
+ * ordinary press; only the time since the focus can.  A quarter second
+ * is far longer than that press and far shorter than any deliberate
+ * stroke made as a separate act after focusing the window.
+ */
+export const FOCUS_SETTLE_MS = 250;
+
+/**
+ * The end-to-end tests turn the focusing-click rule off with
+ * `?no_focus_settle` in the address: a browser driven by WebDriver gains
+ * window focus on the first pointer action after every load or tab
+ * switch, so it cannot tell such a switch from a click that raised the
+ * window, and the rule would refuse legitimate test strokes.  The rule
+ * itself is tested with a dispatched focus event in one tab.  A real
+ * user never sets this, and it disables nothing but this nicety.
+ */
+const focusSettleOn = !new URLSearchParams(location.search).has('no_focus_settle');
+
 /**
  * @typedef {object} PenTarget
  * @property {(clientX: number, clientY: number) => [number, number]} toPage
@@ -100,6 +125,8 @@ export function attachPenInput(surface, target) {
   let pressY = 0;
   /** @type {PointerEvent | null}  The eraser's press, not yet erased with. */
   let eraseWaiting = null;
+  /** When the window last gained focus, for the focusing-click rule. */
+  let lastFocusAt = -Infinity;
 
   /** The eraser's press erases now, if it was waiting. */
   function eraseFromPress() {
@@ -188,6 +215,10 @@ export function attachPenInput(surface, target) {
     if (activePointer !== null) return refuse(`pointer ${activePointer} still active`);
     if (event.target !== surface) return refuse('not on the page');
     if (event.pointerType === 'pen') penSeen = true;
+    // A press that only brought the window to the front draws nothing.
+    if (focusSettleOn && performance.now() - lastFocusAt < FOCUS_SETTLE_MS) {
+      return refuse('it brought the window to the front');
+    }
     // Once a pen has been used, a touch is taken to be the resting hand.
     if (event.pointerType === 'touch' && penSeen) return refuse('a touch after a pen: the resting hand');
     if (event.pointerType === 'mouse' && event.button !== 0) return refuse('not the main button');
@@ -325,7 +356,7 @@ export function attachPenInput(surface, target) {
   // 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('focus', () => { lastFocusAt = performance.now(); trace('window: focus'); });
   window.addEventListener('blur', () => trace('window: blur'));
   document.addEventListener('visibilitychange', () => trace(`visibility: ${document.visibilityState}`));