Issue 760216 has been merged into this issue.

If you are curious about the code, blink::PrintContext::ComputePageRects() and blink::LocalFrame::ResizePageRectsKeepingRatio() may be of interest.

Yeah I've been looking at it. If I use (3000,4000) instead of the document rect in PrintContext::ComputePageRects then everything seems to work as expected.

I think wires are getting crossed somewhere from dealing with logical dimensions, but I'm not quite sure how.

By the way it might also work as expected before this change, I'll double-check tomorrow: https://codereview.chromium.org/1423643002

Correct me if I've made any bad assumptions, but could the issue be with LayoutView::GetLayoutSize?

LayoutView::PageLogicalHeight is described as being used to split the content into pages [1]. Since it's logical, in vertical writing mode it should be the width of the page (makes sense as page breaks are between pages horizontally).

In printing mode LayoutView::GetLayoutSize returns (view width, page logical height) [2].

In vertical mode the Document's LayoutView's physical dimensions are set as follows:
* width is set to the view width from [3].
* height is set to the page logical height from [4]

In horizontal mode the dimensions are instead:
* width is again set to view width from [4]
* height is again set to page logical height from [3]

Note how the height is the logical height in both cases. This means that vertical mode is more like:
* width is set to the view width from [3].
* height is set to the page width from [4]

Which seems wrong... do you think LayoutView::GetLayoutSize should return (page logical height, view height) in the vertical writing case to reflect the page's initial containing block?

But I still don't understand how the change I linked above could have broke it in the first place. Layout code makes my head spin a bit.

[1]
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/LayoutView.h?rcl=0397489124ce7e6aced020f8b85f5034c7d5f49b&l=291
[2]
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/LayoutView.cpp?rcl=bbcbac2763e4d3bd5b1f3a627a89aa0fbfc87523&l=747
[3]
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/LayoutView.cpp?rcl=a9c25d0f8c2e1904a8e4c0baa61c3e58b4258524&l=200
[4]
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/LayoutView.cpp?rcl=a9c25d0f8c2e1904a8e4c0baa61c3e58b4258524&l=204

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