Make amp-carousel always show controls in emails

[fstanis]

Fixes #25713

[sparhami]

I feel like this should be done in a generic way, at some level asking the viewer if they support swipe. There are other components that may need the same treatment. Another option is to enforce the usage of the controls attribute via the validator, with a separate spec that requires it for the email format.

/cc @dvoytenko

[fstanis]

I feel like this should be done in a generic way, at some level asking the viewer if they support swipe.

I like this. Maybe even a doc-level parameter that overrides isMouseDetected? For example, https://amp.example/page#mouse=1 forces that function to return true while https://amp.example/page#mouse=0 forces it to be false?

This not just solves the issue for email (since email clients control the viewer and can choose to override this), but also helps test mouse-related functionality for developers.

Another option is to enforce the usage of the controls attribute via the validator, with a separate spec that requires it for the email format.

This was my first idea, but it breaks existing emails and templates and puts needless burden on developers. I'd rather we solve this on runtime or viewer level if at all possible.

[dvoytenko]

I feel like this should be done in a generic way, at some level asking the viewer if they support swipe.

I'm pretty sure that has already been done at some point before. For instance, don't we always show carousel controls on desktop?

[sparhami]

I feel like this should be done in a generic way, at some level asking the viewer if they support swipe.

I'm pretty sure that has already been done at some point before. For instance, don't we always show carousel controls on desktop?

Carousel 0.1 uses .amp-mode-mouse, but I'm not sure whether adding this class in emails for mobile devices would be the right thing to do. I think we should treat it as @media (pointer: fine). Carousel 0.2 shows the controls until the user changes slides specifically with a touch.

It seems like carousel is the only AMP-internal thing using .amp-mode-mouse, but it seems like sites use it in their CSS. Since email is new, it may be safe to add it in the email format, but I think having a separate mechanism would be preferable.

[fstanis]

For instance, don't we always show carousel controls on desktop?

They don't show on my Linux laptop that has a touchscreen.

but I'm not sure whether adding this class in emails for mobile devices would be the right thing to do

This isn't sufficient I think - I just checked, my laptop has both amp-mode-touch amp-mode-mouse and the controls don't show.

Now that Gmail is rolling out mobile support, I'd merge this PR as a temporary solution for #25713. I'm happy to revert it when we have a better way to do it, e.g. via a viewer capability, but I'd like to avoid carousels breaking on mobile as more users start getting AMP emails there and we haven't communicated proactively enough about this feature.