As I intended to do for a long time, I just committed some more support for documents with viewBox attribute:
If the root <svg> element of your document has width/height attributes set in percentage units _and_ there's a viewBox on that element, then: * The canvas size is set to the value of the viewBox attribute. * Changing the canvas size in Document Preferences sets viewBox, without touching the width/height values.
So now you can visualize the viewBox by the canvas border and even adjust it easily. The only thing you cannot do is switch the document from the "regular type" (absolute-valued width/height, no viewbox) to the "viewbox type" (percentage-valued width/height, viewbox) or back. By default the "regular type" documents are created, as before. I can add such a switch if there's demand for it.
When working on this I discovered that sp_root_write was never called and had lots of rotten code and bugs. I fixed what I could, and it is now used at least when you update document width/height. So please watch for the correctness of the root <svg> in your documents after saving, there may be bugs lurking still.