René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Invalid pixel ratio update when changing screen resolution","articleBody":"### Bug report\r\n\r\n**Bug summary**\r\n\r\nWhen 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.\r\n\r\n**Code for reproduction**\r\n\r\nI am using the [NeXpy package](https://nexpy.github.io/nexpy/), 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:\r\n![external-monitor](https://user-images.githubusercontent.com/1193959/110212397-c7dc5d00-7e60-11eb-85b0-6f3dae41cd48.png)\r\nIn the shell, I see the following ratios.\r\n```\r\nIn [1]: for pv in plotviews:\r\n            print(plotviews[pv].canvas._dpi_ratio, plotviews[pv].canvas._dpi_ratio_prev)\r\n1.0 1.0\r\n1.0 1.0\r\n1.0 1.0\r\n```\r\n**Actual outcome**\r\nWhen I switch to my laptop, the plots look fine initially, but as soon as I select any window, the plot shrinks to quarter size:\r\n\u003cimg width=\"862\" alt=\"laptop-screen\" src=\"https://user-images.githubusercontent.com/1193959/110212448-0bcf6200-7e61-11eb-99fc-bbcbf5e3e072.png\"\u003e\r\n```\r\nIn [2]: for pv in plotviews:\r\n            print(plotviews[pv].canvas._dpi_ratio, plotviews[pv].canvas._dpi_ratio_prev)\r\n2.0 1.0\r\n2.0 1.0\r\n2.0 1.0\r\n```\r\nAfter the plots have shrunk, the `_dpi_ratio_prev` has still not been updated.\r\n```\r\nIn [3]: for pv in plotviews:\r\n            print(plotviews[pv].canvas._dpi_ratio, plotviews[pv].canvas._dpi_ratio_prev)\r\n2.0 1.0\r\n2.0 1.0\r\n2.0 1.0\r\n```\r\n**Expected outcome**\r\nObviously, 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.\r\n```\r\nIn [4]: screen=plotviews['Figure 3'].canvas.window().windowHandle()\r\nIn [5]: plotviews['Figure 3'].canvas._update_screen(screen.screen())\r\nIn [6]: for pv in plotviews:\r\n            print(plotviews[pv].canvas._dpi_ratio, plotviews[pv].canvas._dpi_ratio_prev)\r\n2.0 1.0\r\n2.0 1.0\r\n2.0 2.0\r\n```\r\nI 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`.\r\n\r\n**Matplotlib version**\r\n  * Operating system: Mac OS X 10.15.7\r\n  * Matplotlib version: 3.4.0rc1\r\n  * Matplotlib backend: Qt5agg\r\n  * Python version: 3.8.6\r\n  * PyQt5 version: 5.15.2\r\n\r\nI used pip to update to the latest release candidate within a conda environment.\r\n\r\nEdited to correct reference to the screen.\r\n\r\n","author":{"url":"https://github.com/rayosborn","@type":"Person","name":"rayosborn"},"datePublished":"2021-03-06T16:19:08.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":5},"url":"https://github.com/19656/matplotlib/issues/19656"}