René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: macOS backend flickers/flashes due to use of timers","articleBody":"### Bug summary\n\nRight now, the macOS backend has a lot of \"misfires\" where it will momentary render a blank figure. This typically occurs when the figure is first displayed as well as during a window resize.\n\nHere's a screen recording of me resizing \"galleries/examples/pie_and_polar_charts/polar_demo.py\" on an old MacBook Air:\n\n\u003cdetails\u003e\n\u003csummary\u003eVideo of problem\u003c/summary\u003e\n\n---\n\n\u003cp\u003e⚠️ \u003cb\u003eWARNING\u003c/b\u003e: This video shows a flashing/flickering white box on top of a dark background.\u003c/p\u003e\n\nhttps://github.com/user-attachments/assets/04bbcc99-27ea-4891-974a-c59c5eda25e3\n\u003c/details\u003e\n\nThis also occurs on modern/faster hardware although not as pronounced.\n\nThe root cause is the use of timers to call `setNeedsDisplay:YES`. This was implemented in #21790 in order to fix #10980 and #17445. However, I believe that pull request was done without realizing it's possible to send the Agg buffer directly to an NSView (via layer-backing or layer-hosting) without the need of a `-drawRect:` callback.\n\n---\n\nAt a **very** general level, the macOS run loop has different phases of execution:\n\n1. Event-processing / Timers\n2. Layout\n3. Drawing\n\nRight now, in response to a `draw_idle()` call, the macosx backend creates a single-shot NSTimer and adds it to the run loop. At best, the timer will not fire until the **next** run loop cycle. If we are in \"Phase 1\" above, the earliest that `_draw_idle()` will be called is the **next** \"Phase 1\". We will skip over the actual draw phase (\"Phase 3\") of the current run loop cycle.\n\nAgain, this is a best case scenario. Per [Timer Tolerance](https://developer.apple.com/documentation/foundation/timer?language=objc#Timer-Tolerance):\n\n\u003e \"The system reserves the right to apply a small amount of tolerance to certain timers regardless of the value of the tolerance property.\n\nBasically, using an NSTimer is the equivalent of telling the system: \"perform this invocation no earlier than X, but you can delay a bit after X to save power.\"\n\n`draw_idle()` should instead do the following:\n\n1. If running in the main thread, immediately call `setNeedsDisplay`.\n2. If running on a background thread, `dispatch_async()` to the main thread and then call `setNeedsDisplay`.\n\n\u003e [!CAUTION]\n\u003e This assumes a layer-backed/layer-hosted approach. It won't work with the current `-drawRect:` approach without breaking blitting.\n\n### Code for reproduction\n\n```Python\n# galleries/examples/pie_and_polar_charts/polar_demo.py\n```\n\n### Actual outcome\n\nFlicker, misalignment with macOS drawing phase\n\n### Expected outcome\n\nNo flicker when resizing. No flicker upon first showing the figure.\n\n### Additional information\n\n_No response_\n\n### Operating system\n\nmacOS (Tested on 10.14, 14.7, and 27 beta 2)\n\n### Matplotlib Version\n\n3.11\n\n### Matplotlib Backend\n\nmacosx\n\n### Python version\n\n3.14.5\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\nNone","author":{"url":"https://github.com/iccir","@type":"Person","name":"iccir"},"datePublished":"2026-07-03T17:30:08.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/31991/matplotlib/issues/31991"}