Popup blinks and disappears when size twice map size

[TheTibar]

• [X] I'm reporting a bug, not asking for help
• [X] I'm sure this is a Leaflet.MarkerCluster code issue, not an issue with my own code nor with the framework I'm using (Cordova, Ionic, Angular, React…)
• [X] I've searched through the issues to make sure it's not yet reported

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How to reproduce

• Leaflet version I'm using:[email protected]
• Leaflet.MarkerCluster version I'm using:[email protected]
• Browser (with version) I'm using:Chrome 81
• OS/Platform (with version) I'm using:Windows 10 v1909
• Create markerclusters
• Bind popup on markerclusters
• When height of popup is more than two times the size of the #map div, the popup disappears just after openning

What behaviour I'm expecting and which behaviour I'm seeing

Allow to scroll to read all the popup, even if its size is more than two times bigger than the #map div.

Minimal example reproducing the issue

• [X] this example is as simple as possible
• [X] this example does not rely on any third party code

In marker-clustering-geojson.html from example, simply add this : for(var i = 0; i < 67; i++) { popupText = popupText + 'add line : ' + i + ''; } in onEachFeature (line 46 for example) :

https://jsfiddle.net/4n0qh65c/1/

More info in this jsfiddle : https://jsfiddle.net/3zpwdaL4/

Thanks a lot,

[jfoclpf]

I can confirm that this is a bug, I'm having the same problem (v1.5.0). Did you happen to solve the problem @TheTibar ?

For the maintainers @danzel and @mourner , I made a code test snippet in Stackoverflow such that you can confirm the issue. This happens not only when the popup is bigger than the screen, but also when you move its anchor off screen.

I don't understand why Leaflet.markercluster overrides popups, since without Leaflet.markercluster the bug does not happen. I also put in the SO code snippet an example without Leaflet.markercluster, that is, only standard leaflet popup.