Perf: Only mount SpacingVisualizers when actively visualizing #72800
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Size Change: +25 B (0%) Total Size: 2.45 MB
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Previously, PaddingVisualizer and MarginVisualizer components were always mounted whenever a block supported spacing, causing MutationObservers to run continuously and trigger re-renders on every canvas hover. Now only mounts visualizers when actively hovering padding/margin controls (visualizedProperty is set), preventing unnecessary MutationObserver overhead. Also adds useEffect to force style computation on mount to ensure correct dimensions display when visualizer first appears.
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Flaky tests detected in fd1560a. 🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/19104325350
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getdave
approved these changes
Nov 5, 2025
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getdave
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I tested this with various blocks and the dimensions panel behaved as it did previously.
Reviewing the code this avoids unnecessary subscriptions and avoids even rendering the visualisers if the respective property is not being visualized.
Overall this is an improvement over trunk.
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What?
Previously, PaddingVisualizer and MarginVisualizer components were always mounted whenever a block supported spacing, causing MutationObservers to run continuously and trigger re-renders on every canvas hover.
Now only mounts visualizers when actively hovering padding/margin controls (visualizedProperty is set), preventing unnecessary MutationObserver overhead.
Also adds useEffect to force style computation on mount to ensure correct dimensions display when visualizer first appears.
Why?
We shouldn't be retriggering all these renders when we're not even using the functionality.
How?
Check to see if we want to be visualizing that property, then mount it. When force mounted also update the styles again so we get the correct values.
Testing Instructions
Interact with the dimensions panel on various blocks. There should be no changes from trunk.
Testing Instructions for Keyboard
Screenshots or screencast
Profiling before:
See how many renders hovering is, and how often the spacing visualizers show up.
Screen.Recording.2025-10-29.at.12.45.37.PM.mov
Afterwards, there is just render when hovering. The post locked modal, for some reason... going to look at that one :)
Screen.Recording.2025-10-29.at.12.46.29.PM.mov