Support the `:hover` selector

[pcwalton]

This is needed for Acid2 lines 6, 7, 8, 9 (the nose).

[larsbergstrom]

Needed for #1366.

[larsbergstrom]

Some notes on this at:
https://github.com/mozilla/servo/wiki/Transription--Layout-and-ACID2#pseudo-class

[metajack]

1. Implement Rust event handlers and fix onclick to use them
2. Get mousemove events into script from the compositor
3. Script sets hover state on DOM node (special bit on the element)
4. Selector matching can query that bit
5. Trigger reflow

[jdm]

Step 1 is #1528.

[larsbergstrom]

@sammykim you can work on numbers 2-5 while waiting for Rust event handlers to land. Please ask us in IRC or here on github if you have any questions, particularly about glfw mouse move, hooking into script, and querying the layout task for the correct DOM nodes.

[larsbergstrom]

@sammykim I had a chance to chat with @jdm about some of your questions. Below is a rough outline of the work:

• Set up glfw to receive the mouse position events
• Pass those mouse position events from the compositor to the script task
• The script task should query layout for all of the DOM nodes corresponding to that point on the screen and they should be in a defined order (either bottom-up or top-down is fine - whichever is easiest)
• The script task should trigger the corresponding events, using the features described in issues #1528 and #1527 . The list of mouse events is specified at:
  https://dvcs.w3.org/hg/dom3events/raw-file/tip/html/DOM3-Events.html#events-mouseevents
• For the mouse over event, we should also add a handler that sets the hover state on the DOM node and triggers a reflow if it has changed
• Should ensure that CSS selector matching can properly query that handler bit

The specification for the :hover attribute is here:
http://www.w3.org/TR/CSS21/selector.html#x32

Note that unlike DOM events, which have a more complicated bubbling and cancellation semantics (which is why you need the list of all the DOM nodes for a point on the screen instead of just the top one!), a technically correct implementation of :hover only needs to set the property on the topmost element.

You can implement this work in any order that works for you, and we always prefer seeing partial results so we can give comments before there is too much work in progress. e.g., you could hook up the mouse events and messages and get script handling them but not doing much with them other than querying layout first, before working on the event handling and setting bits.

[hyunjunekim]

@larsbergstrom @jdm
When find element for hover target, Could we use the HitTestQuery with layout_task like LINK?
right?

[larsbergstrom]

@hyunjunekim The HitTestQuery message is almost correct, except that it only returns one item. While that works for the specific case of the :hover pseudo-class, you will need the full list of items to implement the event handler.

[larsbergstrom]

In addition to the EventHandler work mentioned above (which requires #1688 to do), there is one more item. We do not currently parse the selector in ACID2 containing :hover correctly, which I believe is what prevents the patch already landed from working:

60:4 Invalid/unsupported selector: .nose div:hover :before 
61:4 Invalid/unsupported selector: .nose div:hover :after 
63:4 Invalid/unsupported selector: .nose div :after