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According to the xdg_popup protocol, popups can be grabbed
upon touch down, pointer press or keyboard down events. It's
also fine to store a serial of a pointer press event, wait
until pointer released event comes and grab a popup
using the serial before the last event (basically, the one
that came with pointer press).

However, the same technique doesn't work with touch events and
some compositors such as Mutter just dismiss the popup. Some
versions of Weston also do the same.

Previously, we just disabled grabs for gnome environment, but
it's not that correct. There is a better way to handle that.

Instead, store serials from both touch down and up events and
make it possible to check what was the last event. If it was
touch down, do not grab the popup. If the last event was
up, avoid grabbing the popup as long as we can't use the
serial from the event before touch up event came.

Bug: 1123521
Change-Id: If9869271328372a80f55d81a469960f3965f7b6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2385280
Commit-Queue: Maksim Sisov (GMT+3) <msisov@igalia.com>
Reviewed-by: Nick Yamane <nickdiego@igalia.com>
Cr-Commit-Position: refs/heads/master@{#804691}