fix(boot): count the startup timeout only while the window shows

A window behind others or on another Space gets no animation frames, and the
splash waits for two of them before startup finishes. The 30-second timeout kept
running meanwhile and reported a healthy but slow start, such as a cold dev
build, as "OpenIDEA could not start". It now pauses while the document is hidden
and resumes with the time left.

public/boot.js

@@ -142,11 +142,30 @@
     window.removeEventListener("unhandledrejection", onRejection);
     window.removeEventListener("openidea:boot-error", onBootError);
     window.removeEventListener("openidea:boot-ready", onReady);
+    document.removeEventListener("visibilitychange", onVisibility);
   };
-  const timer = window.setTimeout(
-    () => show(rejection === undefined ? text.timeout : `${text.timeout}\n\nUnhandled rejection: ${rejection}`),
-    30_000,
-  );
+  // The timeout counts only while the window shows: a hidden or covered window gets no
+  // animation frames, and the splash waits for those to finish startup.
+  let remaining = 30_000;
+  let armedAt = 0;
+  let timer;
+  const arm = () => {
+    if (failed || ready || timer !== undefined || document.hidden) return;
+    armedAt = Date.now();
+    timer = window.setTimeout(
+      () => show(rejection === undefined ? text.timeout : `${text.timeout}\n\nUnhandled rejection: ${rejection}`),
+      remaining,
+    );
+  };
+  const onVisibility = () => {
+    if (!document.hidden) return arm();
+    if (timer === undefined) return;
+    window.clearTimeout(timer);
+    timer = undefined;
+    remaining -= Date.now() - armedAt;
+  };
+  arm();
+  document.addEventListener("visibilitychange", onVisibility);
   window.addEventListener("error", onError, true);
   window.addEventListener("unhandledrejection", onRejection);
   window.addEventListener("openidea:boot-error", onBootError);

src/platform/boot.test.ts

@@ -95,6 +95,27 @@ it("reports the first rejection when startup then times out", () => {
   expect(details).not.toContain("later rejection");
 });
 
+it("counts the startup timeout only while the window shows", () => {
+  const hidden = (value: boolean) => {
+    Object.defineProperty(document, "hidden", { configurable: true, value });
+    document.dispatchEvent(new Event("visibilitychange"));
+  };
+  try {
+    vi.advanceTimersByTime(20_000);
+    // Behind another window: no frames, so the splash can't finish; that isn't a failure.
+    hidden(true);
+    vi.advanceTimersByTime(60_000);
+    expect(document.getElementById("boot-error")).toBeNull();
+    hidden(false);
+    vi.advanceTimersByTime(9_000);
+    expect(document.getElementById("boot-error")).toBeNull();
+    vi.advanceTimersByTime(1_000);
+    expect(document.querySelector("textarea")?.value).toContain("Startup did not finish within 30 seconds.");
+  } finally {
+    Object.defineProperty(document, "hidden", { configurable: true, value: false });
+  }
+});
+
 it("preserves the first failure", () => {
   window.dispatchEvent(new ErrorEvent("error", { error: new Error("startup failed") }));
   window.dispatchEvent(new ErrorEvent("error", { message: "later failure" }));