EDR-7560 - Throttle rendering when the window is occluded, not just minimized

[Schogol]

The WINDOW_HIDDEN throttle (TriDevice::Throttle: 20ms sleep + ShouldSkipFrame, which renders ~1 frame in 50) is only engaged for a minimized window (WM_SIZE / SIZE_MINIMIZED). A window that is fully covered by another window is not minimized, so it falls through to the weaker WINDOW_OUT_OF_FOCUS path (10ms sleep, no frame-skip) and keeps rendering the full scene for a screen nobody can see.

Measured in-game with a rendered space-view on my machine (NVIDIA GeForce RTX 2080, 60 Hz display, V-Sync on), with the window fully covered by another window:

• DX11: ~64 fps, ~2.3 ms/frame of active rendering
• DX12: ~72 fps, ~3.0 ms/frame

A focused, visible client renders at 60 fps (V-Sync-capped). So with V-Sync on at 60 Hz the client spends more GPU on rendering while the window is not visible than while it is. A covered window bypasses the V-Sync cap and renders above it, so the throttle is effectively backwards for covered windows. A minimized window (only possible in Window mode), by contrast, correctly drops to ~1 rendered frame/sec (~0.5 ms/frame).

DXGI cannot supply the missing signal: DXGI_STATUS_OCCLUDED is not reported for flip-model swapchains (DXGI_SWAP_EFFECT_FLIP_DISCARD, mandatory on D3D12) under DWM composition - both Present() and Present(0, DXGI_PRESENT_TEST) return S_OK while the window is fully covered or minimized. Occlusion is therefore detected from window geometry.

Tr2MainWindow now polls occlusion at 4 Hz (WM_TIMER) and drives the existing WINDOW_HIDDEN throttle:

• IsWindowOccluded() samples 5 points on the window (centre + 4 inset corners) and, at each, finds the topmost opaque top-level window; the window is hidden iff every point is covered, or it is minimized. See-through overlays (WS_EX_LAYERED / _TRANSPARENT / _TOOLWINDOW, e.g. the Alt+Tab switcher) are skipped so they are not counted as occluders.
• Focus-gated: a focused window is on top and cannot be occluded, so it skips the scan and pays only a HasFocus() check; the EnumWindows scan runs only on a backgrounded client.
• Started from window creation, so a client launched straight into the background is throttled immediately.

Result: a covered window now takes the same WINDOW_HIDDEN path as a minimized one, dropping active rendering to ~1 frame/sec while hidden, with no change when focused or visible-but-unfocused.

Disclaimer:
This issue was discovered by me on my local machine and I used Claude to investigate the cause and help me create this fix. I was able to build it fine against Python 3.12 but unable to test on a live client.
I did create a TrinityAL probe which confirmed the occlusion-detection to be working as expected.

This fix should be most noticeable by people running multiple clients on the same machine and lower the overall GPU usage.

[Schogol]

MacOS already has occlusion-detection in Tr2MainWindow_MacOS.mm

void Tr2MainWindow::OnWindowOcclusionChanged_MacOS()
{
	auto window = (NSWindow*)GetWindowID();
	auto visible = ( window.occlusionState & NSWindowOcclusionStateVisible ) != 0;
	gTriDev->SetThrottling( TriDevice::WINDOW_HIDDEN, !visible );
}