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Make GPU hot-add reliable for enabling extra outputs

Pekka Paalanen requested to merge pq/xserver:mr/hotplug-fix into master

When a GPU is auto-bound adding more outputs to a screen, that needs to count as a configuration change on that screen so that a WM listening for RRScreenChangeNotify gets notified and handles it as a hotplug. This is particularly for cases where the outputs are already connected. Otherwise nothing might happen.

Issue #909 (closed) describes a real world case where plugging in a DisplayLink dock with a monitor already connected is sometimes left inactive by GNOME. That issue is a race, and requires adding a sleep(5); as the first thing in NewGPUDeviceRequest() to reproduce reliably. With the sleep, the monitor in the dock will never activate automatically. Add this fix over the sleep, and the issue is gone.

This fix was originally developed on a branch replicating Ubuntu 19.04 patch set based on xserver 1.20.4. Testing on master branch was impossible due to #910 (closed).

Closes: #909 (closed)

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