Fix for lazy loading in free mode

[bcmk]

If both lazy loading and free mode are enabled, lazy loading may not work in some circumstances.
Basically if you move to not loaded yet slides with a mouse and completely stop moving before releasing, slides are not loaded.

I recorded a video showing it: https://youtu.be/zvfhp20sccA.
At first I'm opening a tab with an example built using current master branch of Swiper.
Then I'm opening a tab with the version patched with this proposed change.

Here are examples:
The version from master branch: https://bcmk.github.io/swiper/master.html
Fixed version: https://bcmk.github.io/swiper/fixed.html

I haven't created test cases yet and I would be grateful if someone points me to the correct way to add them.

Corresponding issue: #4274

P.S. It fixes the behavior only for loadOnTransitionStart: true.
The issue for loadOnTransitionStart: false still exists.

[vltansky]

We just started to write cypress tests, thats why autoplay still not exists.
Just create swiper\cypress\integration\modules\autoplay.js file and write the tests there.
See swiper\cypress\integration\modules\core.js for examples and swiper\cypress\support\commands.js for commands.

If you want to help & contribute more you're welcome to include more tests, it would help a lot :)

[bcmk]

@vltansky Thanks for pointing me to the tests! However, I didn't get what autoplay has to do with free mode? I guess I should write my test cases in swiper\cypress\integration\modules\core.js.

[vltansky]

Hey, right, I meant swiper\cypress\integration\modules\lazy-load.js, as it's a module https://swiperjs.com/swiper-api#lazy-loading it should have a separate file.

But if it will help you - you can write the test in swiper\cypress\integration\modules\core.js, I'll organize it later :)

[nolimits4web]

Hi! Thanks for pointing out the issue and for the fix. But this fix introduces some unnecessary calculation and actions running transitions (when there is not). So in 82bcc5c I just introduced new internal event for this, and do the lazy load