SUMMARY
When a user adds a layer style to a layer containing data beyond canvas boundary, Krita doesn't only takes into account the data that appears on canvas. but as soon as you close and reopen the file it applies the layer style to whole content even which is beyond the canvas.

And for layer style I believe it is better to take layer content boundary rather than canvas boundary, as far as I know this is how it works in photoshop. People can move layer to the edges and have some content of the layer go beyond the canvas boundary. 

The issue arises only when adding a layer style, once the document is saved and reopened the layer style works as expected. 



STEPS TO REPRODUCE
1. Open a new document add some square shapes at the edge of the boundary
2. move these square beyond the canvas boundary a bit
3. Add drop dhadow layer style, you will notice the drop shadow appears for the content that is inside the canvas boundary

OBSERVED RESULT
When adding layer style content outside the canvas boundary is ignored

EXPECTED RESULT
Layer boundary instead of canvas boundary should be taken in to account when adding layer style. 

This was reported on KA by a user, you can see the screenshot showing the issue of cut off drop shadow in the topic - https://krita-artists.org/t/drop-shadow-gap-on-edge-of-image-if-a-layers-pixel-data-is-off-canvas/75469