1835432 - Improve coordinate system safety in APZ mochitests

Status: NEW

(Core :: Panning and Zooming, task, P3)

Description

[Botond Ballo [:botond] [away until Jun 24]]

A recurring pain point in writing APZ mochitests has been that our test utilities take their inputs in a variety of coordinate systems, but unlike in C++, we don't have a type system to enforce that we're passing in the right kinds of coordinates.

For example:

• promiseNativeMouseEventWithAPZ() and similar functions that take an aTarget parameter, requires CSS coordinates relative to the target
  • Often we use window as the target, in which case this means CSS coordinates relative to the visual viewport.
• hitTest() requires CSS coordinates relative to the layout viewport
• Direct calls to nsIDOMWindowUtils.sendNative*() require screen coordinates relative to the screen origin
• Element.getBoundingClientRect() returns CSS coordinates relative to the layout viewport

Until we have a more systematic way to express these requirements (for example, by adopting Typescript and using different types for different coordinate systems the way we do in C++), I'd like to suggest that we add a way to express coordinate system annotations as runtime values and check/enforce them using runtime checks.

A concrete way this could work:

• Test helpers that currently accept a pair of coordinates (or a rect etc.) also accept an additional input parameter of e.g. an enum type which specifies what coordinate system the coordinates are in.
• Test helpers either assert that their input is in the coordinate system they expect, or (if possible) convert their input from the specified coordinate system to the one they need.