Book view in M4 does not center images in viewing window

[wlpotter]

When viewing a manifest with behavior of "paged", such as a book or manuscript, the images appear off-center in M4.

Example from the IIIF Cookbook Recipe 9: Simple Manifest - Book.

The first two screenshots are from the M4 demo instance, using the Cookbook manifest.

First, the single 'cover' image (canvas 1) that is centered:

Second, the two-page spread (canvases 2-3), showing that the images not centered in the viewer:

It appears that the second image is added to the right of the first, which is kept centered. I would expect that the gutter between the two images is what is centered in the viewer. This is how M3 worked, as shown the the last screenshot below.

This last screenshot shows the same Cookbook manifest loaded in our M3 instance for the Sinai Manuscripts Digital Library, showing it to have the gutter of the two pages centered in the viewer, as expected:

[wlpotter]

A bit more context here after some testing:

It does not appear to be limited just to book view, as sometimes flipping through images that are individuals causes this issue. This manifest seems to be an example: https://collections.library.yale.edu/manifests/32457253.

Here is a screenshot from the M4 demo instance:

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It may have something to do with M4 not resetting the zoom state when changing canvases. For example:

1. Opening the Cookbook Recipe in M4
2. Clicking the "Next Item" button, which loads items 2/3 as a book view, but off-centered
3. Clicking the "Reset Zoom" button then brings them back into center

[fitnycdigitalinitiatives]

I came across this also, and if you set the osdConfig preserveViewport to false, it should fix it.

I would think that false should probably be the default setting for Mirador because it seems a bit janky otherwise.

[marlo-longley]

Look into connection with #4217