Fix stale time deltas after browser sleep/tab suspension

[jakobbg]

deltaTime and serverDeltaTime are computed once on the first AJAX response and cached for the lifetime of the page. When the browser sleeps or the tab is suspended, the cached values become stale, causing all displayed timestamps to be offset by the sleep duration.

Reset both values to 0 on visibilitychange so Ajax_UpdateTime recalculates them from the next fresh response.

Co-authored-by: AI (Gemini via Antigravity IDE)

[Copilot]

Pull request overview

This PR addresses incorrect UI timestamps after browser sleep/tab suspension by forcing a recalculation of cached client/server time deltas when the page becomes visible again.

Changes:

• Add a visibilitychange listener to reset theWebUI.deltaTime and theWebUI.serverDeltaTime when the tab becomes visible.
• Ensure subsequent AJAX responses recompute time deltas via the existing Ajax_UpdateTime logic.

Suppressed comments (1)

js/rtorrent.js:1386

• js/rtorrent.js is loaded before js/webui.js (see index.html preload order), so visibilitychange can fire while theWebUI is still undefined (e.g., page opened in a background tab). That would throw a ReferenceError and potentially break further initialization. Guard access to theWebUI (or use window.theWebUI) before resetting the deltas.

	if(!document.hidden)
	{
		theWebUI.deltaTime = 0;
		theWebUI.serverDeltaTime = 0;
	}

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[xirvik]

One issue: the test leaks its mock.

beforeEach saves the descriptor from Document.prototype, but the tests define hidden on the instance (document), and afterEach restores only the prototype — the own property is never removed. The second test happens to redefine it, which is why nothing fails today, but a third test that expects real document.hidden would inherit the stub. Adding delete document.hidden; to afterEach fixes it.