Remove viewport estimation from VirtualizingStackPanel #13169
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grokys
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Nov 3, 2023
maxkatz6
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Dec 5, 2023
* Added failing VirtualizingStackPanel viewport test * Remove viewport estimation from VirtualizingStackPanel #Conflicts: # tests/Avalonia.Controls.UnitTests/VirtualizingStackPanelTests.cs
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What does the pull request do?
This PR removes the viewport estimation in
VirtualizingStackPanel, which could be wrong, especially in nested scroll viewers.What is the current behavior?
When inside a
ScrollContentPresenter,VirtualizingStackPanel.EstimateViewport()tries to estimate its viewport when it doesn't have one (it just got attached to the visual tree), and can incorrectly estimate a very large viewport, effectively realizing all items. See #10968 (comment) or the added unit test for a simple reproduction.What is the updated/expected behavior with this PR?
With this PR, we don't try to estimate the parent viewport at all. If we don't have one, it's an empty viewport.
While we can refine the estimation to better handle scrollable containers, I don't think it's worth it. Let me explain:
EstimateViewport, we're in a new measure pass: the arrange pass hasn't run yet, which means the parents bounds might change right after the estimation. In this case, we might still realize totally unneeded items, even with a better estimate.VirtualizingStackPanelgets attached to an already arranged visual tree. An empty viewport means we're doing nothing1, and that is cheap. We can wait to have a proper viewport to realize the items properly.Pinging @grokys to ensure there's nothing obvious I'm missing here.
A unit test has been added.
Fixed issues
Footnotes
well, not exactly, the first item is always realized even with an empty viewport, but that's another issue. ↩