[Octave-bug-tracker] [bug #49053] figure scaling issue with Qt 5 on macOS with Retina / HiDPI scaling

[John Swensen]

Follow-up Comment #68, bug #49053 (project octave):

Setting the scale to 1.0 for FLTK would work for the Linux+HiDPI+FLTK case,
but not for the MacOS+HiDPI+FLTK case. If we set it back to 1.0 for FLTK on
MacOS, the window would be large and the OpenGL drawing small.

It looks like there is a HiDPI fix in FLTK for 1.4.x snapshots after r12249
(from 2017-06-02), but none of the distributions have switched to this yet.

So, for the time being, a complete fix fo the HiDPI problems across multiple
platforms looks like the following:

1) Include the changes to the OpenGL drawing based on the screen_scale as
included in the most recent patch
2) Include the changes to the QTHandles ctor to set the screen scale based on
the devicePixelRatio as included in the most recent patch.
3) (Not yet done) Figure out how to compute the screen_scale from existing
FLTK functions and compute the screen_scale and set it in the FLTK toolkit
ctor *only for Linux HiDPI machines*. It can be left as 1.0 otherwise (even
for MacOS HiDPI).
4) (Not yet done, and not sure if needed because it doesn't use OpenGL,
right?) Figure out how to compute the screen_scale from existing GnuPlot means
and compute the screen_scale and set it in the GnuPlot ctor. 

Any suggested changes to this plan of action?

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