Bug report

Bug summary

When switching from an external monitor, with a dpi ratio of 1.0, to my MacBook Pro laptop, with a dpi ratio of 2.0, existing PyQt windows containing Matplotlib plots look fine after waking from sleep but switch to a quarter size as soon as I select the window, i.e., the fix in the PR #19123 doesn't seem to work in this scenario.

Code for reproduction

I am using the NeXpy package, which embeds Matplotlib canvases in separate PyQt5 windows. Each window is added to a dictionary, called plotviews, which is available to an embedded Python shell. If I initially plot on my external monitor, I get the following plot:

In the shell, I see the following ratios.

In [1]: for pv in plotviews:
            print(plotviews[pv].canvas._dpi_ratio, plotviews[pv].canvas._dpi_ratio_prev)
1.0 1.0
1.0 1.0
1.0 1.0

Actual outcome
When I switch to my laptop, the plots look fine initially, but as soon as I select any window, the plot shrinks to quarter size:

In [2]: for pv in plotviews:
            print(plotviews[pv].canvas._dpi_ratio, plotviews[pv].canvas._dpi_ratio_prev)
2.0 1.0
2.0 1.0
2.0 1.0

After the plots have shrunk, the _dpi_ratio_prev has still not been updated.

In [3]: for pv in plotviews:
            print(plotviews[pv].canvas._dpi_ratio, plotviews[pv].canvas._dpi_ratio_prev)
2.0 1.0
2.0 1.0
2.0 1.0

Expected outcome
Obviously, the plots should not change size. Selecting the window does not appear to trigger the canvas showEvent function. I'm not sure what does. If I force an update, the plot does expand to the correct size. Note that _dpi_ratio_prev has been updated after the explicit call to _update_screen but only for the third figure. The others are still shrunk.

In [4]: screen=plotviews['Figure 3'].canvas.window().windowHandle()
In [5]: plotviews['Figure 3'].canvas._update_screen(screen.screen())
In [6]: for pv in plotviews:
            print(plotviews[pv].canvas._dpi_ratio, plotviews[pv].canvas._dpi_ratio_prev)
2.0 1.0
2.0 1.0
2.0 2.0

I wonder if the screenChanged signal should be connected to _update_screen at an earlier stage in initializing the window without depending on a call to showEvent.

Matplotlib version

• Operating system: Mac OS X 10.15.7
• Matplotlib version: 3.4.0rc1
• Matplotlib backend: Qt5agg
• Python version: 3.8.6
• PyQt5 version: 5.15.2

I used pip to update to the latest release candidate within a conda environment.

Edited to correct reference to the screen.