test_driver.click() is unreliable because of coordinate rounding issues

[smfr]

test_driver.click() determines if an element is targetable by computing its client rect, truncated to the viewport, and calling elementsFromPoint() at the center of the rect. If that finds the element, then it dispatches an event using those center coordinates.

The problem is that client rects, and elementsFromPoint(), support fractional x and y values, but mouse events do not (x and y get floored). The code can thus think that an element is in view, but a dispatched event fails to hit it.

In the example I investigated, the target element's top was 599.5px in 600px tall viewport, so the center point was y=599.75 and elementsFromPoint found the element, but the event dispatch at y=599 failed to hit it.

Fix by ensuring that at least 1px of the element is visible by adding rectConstrainedToViewport().

Use 'const' and 'let' in this code.

Rename the confusing getPointerInteractablePaintTree to getElementListAtVisibleCenter.

Fixes #53239

[gsnedders]

See comments below; also cc @masayuki-nakano, @mustaqahmed, @smaug----.

We probably need to make sure this doesn't affect any of the Pointer Events or UI Events tests, at least.

[gsnedders]

Well, them flooring to integers depends on which spec you're looking at, given https://w3c.github.io/uievents/#mouseevent and https://drafts.csswg.org/cssom-view/#extensions-to-the-mouseevent-interface give differing definitions for the same attributes. (See w3c/uievents#24 & w3c/csswg-drafts#4084.)

https://w3c.github.io/pointerevents/#event-coordinates also says "user agents that have implemented the proposed change in CSSOM View Module only for PointerEvent", which implies this doesn't apply to all user agents.

I'm mostly slightly weary about making it impossible to test the Pointer Events behaviour here by introducing rounding always.

Something similar to what we do in this pointer events test might work, checking that at least 1 pixel is in view when fractional values aren't supported, and checking greater than 0 pixels are in view when they are:

wpt/pointerevents/pointerevent_fractional_coordinates_untrusted.html

Line 107 in c740cb6

if ((new PointerEvent("click", {screenX: 0.1})).screenX == 0.1) {

[mustaqahmed]

https://w3c.github.io/pointerevents/#event-coordinates also says "user agents that have implemented the proposed change in CSSOM View Module only for PointerEvent", which implies this doesn't apply to all user agents.

If fractional coordinates in click a concern here (I am not sure), we do not need to support that to the best of my knowledge:

• IIRC, "user agents that have implemented..." was added to cover the unclear "requirement" around CSSOM-View extension: fractional coordinates are neither required not optional because of Converge with CSSOM-View w3c/uievents#24!
• We (Blink) already confirmed that non-integer coordinates are not web compatible for click-like events.

[gsnedders]

This function was intended to match https://w3c.github.io/webdriver/#dfn-pointer-interactable-paint-tree, and named as such.

I'm not saying we can't deviate from WebDriver here, but it does hint at there maybe being issues in the WebDriver definitions here.