@fnicollet The unit tests are failing with the same symptoms I (and @yohanboniface) noticed today - we might need to check the popup offset manually before merging.

Yes, I would say that this commit is the source of the unit test failing:
bddfb11

Ok, I think I am beginning to understand. Before my PR, the tests fail when zoomAnimated is true, but succeed when zoomAnimated is false. The test is not just one the Popup's zoomAnimation :

this._zoomAnimated = this._zoomAnimated && this.options.zoomAnimation;

but also on "this._zoomAnimated", which comes from Layer.js:
https://github.com/Leaflet/Leaflet/blob/master/src/layer/Layer.js#L86

So it takes in account map's zoomAnimation. So my guess is that when running the tests in phantom, the map is set as not animated, probably this test is failing in that env:
https://github.com/Leaflet/Leaflet/blob/master/src/map/anim/Map.ZoomAnimation.js#L18

So basically, the tests have been written for the zoomAnimation=false edge case and not the zoomAnimation=true default case.

So I would say, let's fix the tests instead?

Should this test be removed?

It tests for marker1._popup._container.style.left, which now always depends on the popup's width/height but doesn't give any indication of the popup's position relative to its anchor
Because we are now in the zoomAnimated case, the popup's position is changed via "L.DomUtil.setPosition", which changes the CSS transform property and not the left/bottom properties.

So I would ditch this test. Tell me if you want me to add it to the PR

@fnicollet popup positioning would certainly deserve a bit of cleaning, but AFAIK even with your commit we are setting the bottom and left manually. So this test should pass IMHO.
Certainly, even when in zoomAnimated, we should take care of updating the offset with the anchor.

Though I think we may have an issue when L.Browser.any3d is false.

Here is what I understood from the code:
This specific Spec tests the facts that popupAnchor is well interpreted. It does so by comparing the position of a popup with and without a "popupAnchor" on the icon.

• "popupAnchor" has an effect on _getAnchor, which drives the CSS transform of the container
• "offset" has no relationship with popupAnchor, it just affects the bottom/left properties of the container.

Both combined achieve the final placement of the popup. So if the test really wants to test popupAnchor, it should be changed to a test which account for both (using element.offsetLeft or something?) or removed

Just saw your other comment, there will indeed be an issue if L.Browser.any3d is false as setPosition will change top left, and then it will set left/bottom right after and override what's been done in setPosition.

Unable to merge due to conflicts with the new tooltip/divoverlay code. Superseded by #4685