Listen to screen DPI changes

[jwiggins]

Closes #863

Note that I haven't tested this extensively. base_pixel_scale gets cached in a few places, so there a probably lingering corner cases.

[rahulporuri]

CI is failing because we changed base_pixel_size into a readonly property but a test tries to set it -

======================================================================
ERROR: test_component_hidpi_size_stability (enable.tests.test_component_editor.TestComponentEditor)
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Traceback (most recent call last):
  File "/home/runner/.edm/envs/enable-test-3.6-pyqt5/lib/python3.6/site-packages/enable/tests/test_component_editor.py", line 86, in test_component_hidpi_size_stability
    dialog.thing.window.base_pixel_scale = 2.0
  File "/home/runner/.edm/envs/enable-test-3.6-pyqt5/lib/python3.6/site-packages/traits/trait_type.py", line 67, in _read_only
    % (name, class_of(object))
traits.trait_errors.TraitError: The 'base_pixel_scale' trait of a Window instance is 'read only'.

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[corranwebster]

Revisiting this, it looks like the only place we are setting the base_pixel_scale is here:

enable/enable/tests/test_component_editor.py

Line 86 in 718c863

dialog.thing.window.base_pixel_scale = 2.0

[corranwebster]

This works really smoothly when moving between HiDPI laptop and regular resolution external monitor. Manually tested using:

• agg
• celiagg
• qpainter
• quartz
• gl

The gl backend had issues with HiDPI display, but other than that everything worked as expected.

[corranwebster]

I have opened #907 for the OpenGL issues.

[jwiggins]

Wait... How is the Wx test passing now?

[corranwebster]

Wait... How is the Wx test passing now?

Two tests - one for Wx (the old one), one for Qt

[jwiggins]

But that doesn't explain why setting base_pixel_scale in the Wx test doesn't raise a TraitError. What am I missing?

[corranwebster]

But that doesn't explain why setting base_pixel_scale in the Wx test doesn't raise a TraitError. What am I missing?

That the original change was on the Qt-specific Window subclass and not the Wx one. This PR doesn't attempt to handle the Wx case, I think.

[jwiggins]

Right... Thanks for helping me finally understand the code that I wrote. 😞

And thanks for seeing this change through! LGTM.