fix(reader): stop tap zones from paging during long-press text selection

Selecting text near the left edge on a touch browser could page back
instead: a long-press released just inside the 500ms tap window (Android
selection engages at ~400-500ms, right at the guard boundary) or landing
on a margin resolved as a tap, armed the 280ms debounce, and nothing
ever cancelled it. Four hardenings in the tap-zone pipeline:

- contextmenu (Android's long-press-engaged signal) suppresses the
  matching pointerup from counting as a tap, closing the duration race
- pointercancel (the browser taking over the gesture) now resets the
  tracked pointer so stale state can never match a later touch
- selectionchange on the host document and every content iframe cancels
  an armed tap action: a selection appearing right after finger-lift
  means the 'tap' was a long-press selection engaging
- the host-viewport pointerup honors the tracked any-selection flag,
  closing the blind spot where selections in iframes or the host's own
  fixed-layout text layer were invisible to the host surface (the
  per-surface check only ran for iframe docs)

Purely touch-path (coarse pointer) changes; desktop behavior untouched.

web/src/reader/reader.ts

@@ -406,6 +406,10 @@ document.addEventListener("alpine:init", () => {
     tapZonesEnabled: true as boolean,
     tapZoneSize: 30 as number,
     tapZoneTimer: null as ReturnType<typeof setTimeout> | null,
+    // Any live text selection, host document or content iframe. Fed by
+    // selectionchange listeners (touch devices); tap zones stand down
+    // while one exists.
+    anySelection: false as boolean,
     highlightItems: [] as {
       id: string;
       text: string;
@@ -691,6 +695,14 @@ document.addEventListener("alpine:init", () => {
         // out to the host document, so the viewport listeners miss them).
         if (window.matchMedia("(pointer: coarse)").matches) {
           this.attachTapZoneListeners(doc as unknown as HTMLElement, true);
+          // Same for selectionchange: a selection inside the iframe must
+          // cancel armed tap actions and feed the host-surface guard.
+          doc.addEventListener("selectionchange", () => {
+            const sel = doc.getSelection();
+            this.noteSelectionActivity(
+              !!sel && !sel.isCollapsed && !!sel.toString(),
+            );
+          });
         }
         // Text selection → highlight popover (reflowable EPUB only;
         // fixed-layout highlight overlays are a later milestone).
@@ -991,6 +1003,26 @@ document.addEventListener("alpine:init", () => {
       if (!window.matchMedia("(pointer: coarse)").matches) return;
       const vp = document.getElementById("reader-viewport");
       if (vp) this.attachTapZoneListeners(vp as HTMLElement, false);
+      // Host-document selections (fixed-layout/PDF text layers, margins):
+      // selectionchange never crosses iframe boundaries, so register per
+      // surface.
+      document.addEventListener("selectionchange", () => {
+        const sel = document.getSelection();
+        this.noteSelectionActivity(
+          !!sel && !sel.isCollapsed && !!sel.toString(),
+        );
+      });
+    },
+    // Record selection state and abort any armed tap action: a selection
+    // appearing right after finger-lift means the "tap" was actually a
+    // long-press selection engaging, and paging away would destroy the
+    // gesture the user just made.
+    noteSelectionActivity(hasSelection: boolean) {
+      this.anySelection = hasSelection;
+      if (hasSelection && this.tapZoneTimer) {
+        clearTimeout(this.tapZoneTimer);
+        this.tapZoneTimer = null;
+      }
     },
     attachTapZoneListeners(surface: HTMLElement, isDoc: boolean) {
       let downX = 0;
@@ -998,6 +1030,12 @@ document.addEventListener("alpine:init", () => {
       let downT = 0;
       let downId = -1;
       let moved = false;
+      // Android fires contextmenu when a long-press engages text
+      // selection: that press must never resolve into a tap action, even
+      // when it was released inside the 500ms tap window (the selection
+      // engaging and the guard racing is exactly how corner selections
+      // used to page back instead).
+      let longPressed = false;
       surface.addEventListener(
         "pointerdown",
         (e: PointerEvent) => {
@@ -1007,6 +1045,21 @@ document.addEventListener("alpine:init", () => {
           downT = Date.now();
           downId = e.pointerId;
           moved = false;
+          longPressed = false;
+        },
+        { passive: true },
+      );
+      surface.addEventListener("contextmenu", () => {
+        longPressed = true;
+      }, { passive: true });
+      // The browser takes over the gesture (text selection, scroll) with
+      // pointercancel — no pointerup will follow. Drop the tracked
+      // pointer so stale state can never match a later touch.
+      surface.addEventListener(
+        "pointercancel",
+        () => {
+          downId = -1;
+          moved = false;
         },
         { passive: true },
       );
@@ -1024,7 +1077,7 @@ document.addEventListener("alpine:init", () => {
         (e: PointerEvent) => {
           if (e.pointerId !== downId) return;
           downId = -1;
-          if (moved || Date.now() - downT > 500) return;
+          if (moved || longPressed || Date.now() - downT > 500) return;
           if (!this.tapZonesEnabled) return;
           const target = e.target as HTMLElement | null;
           if (
@@ -1035,6 +1088,10 @@ document.addEventListener("alpine:init", () => {
             return;
           const sel = isDoc ? (surface as any).getSelection?.() : null;
           if (sel?.toString?.()) return;
+          // Host-surface blind spot: selections living in content iframes
+          // (or the host's own fixed-layout text layer) never show in a
+          // per-surface check — the tracked flag covers them.
+          if (!isDoc && this.anySelection) return;
           // No tap actions while a fixed-layout page is zoomed — taps then
           // belong to the content (and double-tap zoom).
           if (this.isFixedLayout && this.renderer?.zoom != null) return;