Rendering problems in Firefox

[ahankinson]

There is a problem with rendering pages with firefox. Symptoms are flickering canvas and an error in the console:

"IndexSizeError: Index or size is negative or greater than the allowed amount"

This can be seen with this manifest:

https://digilib.mpiwg-berlin.mpg.de/digitallibrary/Manifester/IIIF/permanent!library!KNA1AZYB!pageimg

[robcast]

I can also reproduce the exception with the default test manifest in a fresh build of diva.js master after some scrolling in Firefox (64b dev).

[robcast]

To me it looks like destHeight is set to zero when scaledTile.dimensions.height or the other one are equal to destYOffset in:

diva.js/source/js/renderer.js

Line 392 in c8a703e

const destHeight = Math.min(scaledTile.dimensions.height, img.height * scaledTile.scaleRatio) - destYOffset;

I don't know what this line is supposed to do and why it malfunctions but I get rid of the exceptions when I skip the drawImage() when the computed widths or heights are zero.

This is not a real fix of course ;-)

[ahankinson]

that actually could be a solution; if the tile is less than 1, we probably should not attempt to draw it.