fix for Scattergl animation bug

[alexturcea]

trafficstars

closes https://github.com/plotly/plotly.js/issues/6251

[alexturcea]

If anyone has time to double check these changes it would be very helpful. Essentially, there are a few issues that this fix should address:

1. when using scattergl with a multiframe plot (i.e. an animation), the scatterplot is only visible on one frame
2. even if the above is fixed (see my first suggested fix), when the number of plots is changing across frames, the plot is not updating correctly.
3. similar to 2. , when the number of annotations is changing across frames, the plot is not updating correctly

@alexcjohnson @archmoj @jonmmease

[alexturcea]

Code to reproduce: https://jsfiddle.net/gcptmL6v/

With the proposed fix: https://jsfiddle.net/q2wt06d3/

[alexcjohnson]

@alexturcea great! Nicely done, and your fiddles are very convincing. I'll let @archmoj give the code a review, but we'll need to adapt your fiddle to a test we can run on CI - maybe in animate_test.js? And hopefully we can pare it down to something small and readable 😉

A bunch of failures in the CI runs, but they don't look like the kind of thing that would have resulted from your changes.

[alexturcea]

Hi @alexcjohnson @archmoj , I synched this with the main branch, still hoping to have it merged 😄

[alexcjohnson]

Thanks @alexturcea - as I mentioned before, we'll need to adapt your fiddle into a test, would you be able to try that? We'll also need a draftlog item.

[archmoj]

Please fetch upstream/master and merge in the branch. Then please update your https://jsfiddle.net/q2wt06d3/ using a build from publish-dist/artifacts. Thank you!

[archmoj]

@alexturcea Are you interested in completing this PR? Could you try @eiriklv suggestion? https://github.com/plotly/plotly.js/pull/6452#discussion_r1630042980