Leaflet .getSize() issue with Svelte

[gavinr]

Describe the bug
When creating a leaflet map, the leaflet getSize() function sometimes returns the wrong size within a Svelte app.

To Reproduce

1. Clone this repo: https://github.com/gavinr/leaflet-custom-layer-issue
2. npm install
3. npm run build
4. npm start

in a CHROMIUM-based browser (example: google chrome), open http://localhost:5000/ and refresh/hard-refresh a bunch of times until you see the "y" value in the developer console be an unreasonably small (~30 or 40) size.

Expected behavior

• Expected: Size in the developer console should always be the same
• Actual: sometimes on new refreshes the y value of the size is not correct (a very small number)

[antony]

I'm not sure how this is an issue with Svelte. It seems that you're trying to get the size of a layer before it has properly rendered. It's a race condition.

[gavinr]

@antony thank you for the response. This works fine when not using Svelte - see https://github.com/gavinr/leaflet-custom-layer-issue/blob/master/public/standalone.html (sorry, should have included this in the original issue report), which is what makes me think it's an issue with Svelte.

[gavinr]

It seems like the DOM or the styles sometimes have not fully loaded when this function gets called. Note I am using svelte actions here to call the function. When I use an action like this can I assume the DOM and styles are loaded? If not, what's a better way to guarantee that the the DOM and styles are loaded?

[Conduitry]

You should be able to assume that the DOM and any styles are loaded, yes, but that's not to say that instantiating the LeafletMap instance has already synchronously done everything it's going to do to the DOM and that its getSize() method will return the desired values right away. If an element's size is affected by whether an image within it has finished downloading, that may also give you unexpected values. You're free to open another issue that shows something specifically wrong happening in Svelte, but for now, it looks like this issue involves you instantiating an external library and then after that calling a method on that external library and having an issue with its return value.