D&D fails sometimes, especially when Firefox main loop is busy.

It's because we reply to drag_motion callback too late (we reply from scheduled timer event) and Gtk fails to deliver / commit reply to Wayland compositor. We definitely violate D&D protocol here.

Primary cause of this bug is missing reply from drag_motion handler while we're reading data from D&D and routing it to child process.

Wayland/Gtk sets drag state right after drag_motion event only. We're getting 'drag_motion' events from nested loop (while reading data for child process) and we don't know the final D&D state yet as we need reply from the child process. When we set the D&D state latter, it's ignored by Gtk and D&D is considered as canceled by compositor.

Right now we explicitly state 'drag denied' after D&D start until we have a reply from child process. We may change it to 'drag accepted' on Wayland. It does not change final D&D state - we get the data only when child process requests it.

When we get data from D&D operation by nsWindow::OnDragDataReceivedEvent, speed up data transfer to child process by
WindowDragMotionHandler() call.

Wayland/Gtk sets drag state right after drag_motion event only.

While we're reading data for child process we're getting 'drag_motion' events from nested loop.
We don't know the final D&D state yet as we need reply from the child process.
When we set the D&D state latter, it's ignored by Gtk and D&D is considered as
canceled by client.

Right now we explicitly state 'drag denied' after D&D start until we have a reply from child process.
We're changing it to 'drag accepted' on Wayland until we have a reply from child process
and we also send the reply in WindowDragMotionHandler() to fulfill Wayland protocol needs.

It does not change final D&D state - we get the data only when child process requests it.

Backed out for causing linux bustages on nsWindow.cpp.

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=b0abd344cf28b55c2c52c180271de3d88143976c&selectedTaskRun=U0TtTmy3Sr-i8PFG9zA0fQ.0

Failure log: https://treeherder.mozilla.org/logviewer?job_id=352246192&repo=autoland

Backout link: https://hg.mozilla.org/integration/autoland/rev/49ce22ba1d18270228c9d79317aa28571dfbe2c4

Let's skip this one.

https://hg.mozilla.org/mozilla-central/rev/11250511cdb8

Backed out along with Bug 1731197 for drag and drop of tabs causing duplicates (Bug 1732116).

backout: https://hg.mozilla.org/mozilla-central/rev/ba378c62cecf6a3048baddde1f9c7d61b45a2914

No need to backout that, a fix is already at Bug 1732116

https://hg.mozilla.org/mozilla-central/rev/ea39b67162eb