Treat idle timeout 0 as disabled, not immediate

idle.screensaver / idle.lock of 0 was included in Math.min(), so
IdleMonitor's timeout became 0. Releasing a Wayland idle inhibitor
(SDL's default screensaver inhibit) then reported idle immediately and
locked the session even when lock had been "disabled" by setting it to 0.

Treat 0 as disabled when computing the first idle deadline, do not start
the corresponding action, and leave IdleMonitor off when both timeouts
are 0.

Fixes #10860

shell/plugins/services/idle/IdleModel.js

@@ -4,6 +4,26 @@ function secondsFromConfig(value, fallback) {
   return Math.floor(n)
 }
 
+// 0 means the action is disabled, not "fire immediately". min(0, 300) would
+// otherwise make IdleMonitor report idle as soon as an inhibitor is released.
+function firstIdleTimeout(screensaverSeconds, lockSeconds) {
+  var times = []
+  if (screensaverSeconds > 0) times.push(screensaverSeconds)
+  if (lockSeconds > 0) times.push(lockSeconds)
+  if (times.length === 0) return 0
+  var min = times[0]
+  for (var i = 1; i < times.length; i++) {
+    if (times[i] < min) min = times[i]
+  }
+  return min
+}
+
+function delayAfterFirstIdle(timeoutSeconds, firstIdleSeconds) {
+  if (!(timeoutSeconds > 0)) return 0
+  var delay = timeoutSeconds - firstIdleSeconds
+  return delay > 0 ? delay : 0
+}
+
 function eventParts(event, count) {
   try {
     if (event && event.parse) return event.parse(count)
@@ -46,6 +66,8 @@ function screensaverWindowsAfter(windows, address, visible) {
 if (typeof module !== "undefined") {
   module.exports = {
     secondsFromConfig: secondsFromConfig,
+    firstIdleTimeout: firstIdleTimeout,
+    delayAfterFirstIdle: delayAfterFirstIdle,
     eventParts: eventParts,
     screensaverWindowsAfter: screensaverWindowsAfter
   }