There is a problem with the zoom in DocViewer. As one zooms in the DocViewer scrolls up the document. For example:
I start on page 7 of the document fully zoomed out
I zoom in one step and now I am on page 6
I zoom in another step and now I am on page 5
Another step and I am looking at the bottom of page 4 and the top of page 5
Another step and now I can only see the bottom of page 4
Another step and I can see only the top of page 4
Another step and I see only the bottom of page 3
As one zooms out this is reversed.
After contacting Canvas support I have been told that this is the intended behaviour. Two people at Canvas support have told me the same thing: the focal point of the zoom is the top left corner (the top left corner of what I am not sure).
The following video explanation of one of the Canvas support engineers shows exactly what the problem is: VIDEO. (I am perplexed that the engineer giving the explanation seems to think the behaviour of the zoom tool she is demonstrating is reasonable.)
The sensible behaviour would be for the focal point of the zoom to be the top left corner of the currently visible part of the document. This is how most (if not all) PDF viewers' zooms work. It keeps the topmost and leftmost part of the visible page in the top left corner of the window when zooming in and out. I have been told I have to make a feature suggestion to fix the buggy/nonsensical behaviour of the zoom, so this is what I am doing:
My feature suggestion is:
Fix the zoom in DocViewer so it behaves like those in most PDF viewers (e.g. Adobe Acrobat Reader DC, SumatraPDF).