PR summary

When we upscale the DPI for high-dpi screens we stash the original dpi and then set the figure dpi to the scaled version. If the same Figure instance is repeatedly passed to a Canvas that support hi-dpi it would go into a loop where the scaled DPI is treated as the original dpi and infinitely increases.

By grabbingfig._original_dpi (which we stash for exactly this reason) we can avoid this loop.

closes#26380

This is a slightly more general version of what we had already done to support re-attaching Figures to pyplot.

This is covered by the tests for dpi ratio tests we added as part of re-attaching to Figures to pyplot (which my first version of this broke so I'm sure it is being hit).

There is a case that we should move the "re run with device ratio 1" logic intoCanvasBase, but any backend that does support scaling will take care of re-scaling the canvas on display so setting it back to 1 inCanvasBase.__init__ seems like it will always be redundent for users and we have a hook to explicitly clean it up when we remove the Figure from pyplot.

I suspect there is still a few more iterations before we have covered all the stress-cases on this, but this is another step in the right direction.

PR checklist

• "closes #0000" is in the body of the PR description tolink the related issue
• new and changed code istested
• [N/A]Plotting related features are demonstrated in anexample
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