Masayuki Nakano 7653ddaefd Bug 1528289 - part 2: Dispatch same events on the web contents when autoscroll is canceled with a click r=Gijs,edgar
Chrome behaves like this:

1. When user starts autoscroll with a middle click, `mousedown` and `mouseup`
   are fired, but `auxclick` nor `paste` event is not fired.
2. When user ends autoscroll with a left click, only `mouseup` event is fired.
   I.e, `mousedown` nor `click` event is not fired.
3. When user ends autoscroll with a middle click, only `mouseup` event is fired.
   I.e., `mousedown`, `auxclick` nor `paste` events is not fired.
4. When user ends autoscroll with a right click, `mouseup` and `contextmenu`
   events are fired, but `mousedown` and `auxclick` events are not fired.

This patch emulates these Chrome's behavior as far as possible.  However,
unfortunately, we cannot do exactly same behavior without some big patches
because each widget (`nsWindow` or `nsChildView`) discards a mouse event
which rolled up a widget before dispatching it into the DOM.  Therefore,
for now, this patch does not fix the following issues:

1. `mousedown` event is not fired in content when clicking outside the
   autoscroller to close it except when pressing the secondary button or on any
   buttons on Linux.
2. `mouseup` event is not fired in content when clicking outside the
   autoscroller to close it except when pressing the primary button macOS.
3. `click` event and `auxclick` events are fired when clicking outside the
   autoscroller with the secondary button.

So, the middle button `click`/`auxclick` events and `paste` event which is
reported to the bug won't be fired with this patch.  I'll file follow up bugs.

Differential Revision: https://phabricator.services.mozilla.com/D104652
2021-03-23 19:22:48 +00:00

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