On platforms that build with PAN_SCROLLING enabled (this includes all Windows ports), mousedown events aren't fired while pressing and holding another mouse button. For example, suppose you press and hold on an DOM element with the left mouse button and then press the right mouse button. Disregarding mousemove events, the following events are fired (in order): mousedown (left mouse button). But, we should have fired the following events (in order): mousedown (left mouse button), mousedown (right mouse button).
Created attachment 89867 [details] Patch and layout test While working on this patch I noticed that we could increase our test coverage for pan scrolling. I filed bug #58702 for one such layout test we could add.
Created attachment 89868 [details] Self-contained Layout Test For convenience, a self-contained version of the layout test included in the patch.
Comment on attachment 89867 [details] Patch and layout test Clearing flags on attachment: 89867 Committed r84217: <http://trac.webkit.org/changeset/84217>
All reviewed patches have been landed. Closing bug.