fix(grid): register assigned definition collections with their shared size group - #21848
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MrJul merged 5 commits intoJul 26, 2026
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Definitions supplied through the ColumnDefinitions or RowDefinitions setter are already in the collection when the grid claims it, so they never pass through the collection-changed handler that joins them to the parent tree. They never register with their shared size group, and the definitions they replace never unregister from it.
DefinitionList.SetParent assigned each definition's Parent but never called OnEnterParentTree, which only ran from the collection-changed handler. Assigning Parent is not sufficient: OnEnterParentTree also sets InheritanceParent, and a definition cannot read the inherited PrivateSharedSizeScope that registers it with its group until that link exists. Definitions supplied through the ColumnDefinitions setter - an object initializer, a shared resource, or ColumnDefinitions="Auto,*" - were therefore silently absent from their shared size group. Enter and exit the parent tree from SetParent, and release the outgoing collection when Grid swaps one in. Without that release the replaced definitions stay registered with the group; nothing resets their measured minimum any more, so they pin it at whatever they last contributed.
MrJul
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backport-candidate-12.1.x
Removing a definition leaves it holding its old Parent and its property inheritance link, so it still reads the grid's shared size scope and can re-register itself into a scope it has left. Also covers moving a definition between grids, reassigning the same collection, and row definitions, which the assignment fix reached but nothing exercised.
Definition ownership was implemented twice, and the two paths disagreed: SetParent exited a definition and cleared its Parent, while removing one from the collection called OnExitParentTree but left Parent set. Detach was incomplete either way, since OnEnterParentTree establishes InheritanceParent but OnExitParentTree never cleared it - so a removed definition kept reading the grid's inherited PrivateSharedSizeScope, and the grid kept it alive as an inheritance child. Route every owner change through one transition that exits the old tree, assigns Parent, and enters the new one, and clear InheritanceParent on exit so detach mirrors attach.
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You can test this PR using the following package version. |
…st-setparent # Conflicts: # tests/Avalonia.Controls.UnitTests/GridTests.cs
MrJul
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You can test this PR using the following package version. |
MrJul
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… size group (#21848) * test(grid): reproduce shared size groups ignoring assigned definitions Definitions supplied through the ColumnDefinitions or RowDefinitions setter are already in the collection when the grid claims it, so they never pass through the collection-changed handler that joins them to the parent tree. They never register with their shared size group, and the definitions they replace never unregister from it. * fix(grid): join assigned definition collections to the parent tree DefinitionList.SetParent assigned each definition's Parent but never called OnEnterParentTree, which only ran from the collection-changed handler. Assigning Parent is not sufficient: OnEnterParentTree also sets InheritanceParent, and a definition cannot read the inherited PrivateSharedSizeScope that registers it with its group until that link exists. Definitions supplied through the ColumnDefinitions setter - an object initializer, a shared resource, or ColumnDefinitions="Auto,*" - were therefore silently absent from their shared size group. Enter and exit the parent tree from SetParent, and release the outgoing collection when Grid swaps one in. Without that release the replaced definitions stay registered with the group; nothing resets their measured minimum any more, so they pin it at whatever they last contributed. * test(grid): cover the definition ownership contract Removing a definition leaves it holding its old Parent and its property inheritance link, so it still reads the grid's shared size scope and can re-register itself into a scope it has left. Also covers moving a definition between grids, reassigning the same collection, and row definitions, which the assignment fix reached but nothing exercised. * refactor(grid): centralise definition parent-tree transitions Definition ownership was implemented twice, and the two paths disagreed: SetParent exited a definition and cleared its Parent, while removing one from the collection called OnExitParentTree but left Parent set. Detach was incomplete either way, since OnEnterParentTree establishes InheritanceParent but OnExitParentTree never cleared it - so a removed definition kept reading the grid's inherited PrivateSharedSizeScope, and the grid kept it alive as an inheritance child. Route every owner change through one transition that exits the old tree, assigns Parent, and enters the new one, and clear InheritanceParent on exit so detach mirrors attach. --------- Co-authored-by: Julien Lebosquain <julien@lebosquain.net>
MrJul
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RedQuE3n
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Aug 15, 2026
…the guard that could not have noticed LunaTable put every column in a shared size group and set the shared size scope on its root, which is Avalonia's own mechanism for making an Auto column in the header line up with the Auto column in each row. The wiring was right and it never did anything. Avalonia 12.1.0 registers a definition with its shared size scope when the definition is ADDED to the collection a Grid already owns, and does not when a ready-made collection is ASSIGNED to the Grid. LunaTable assigned, in both places - the header grid in Rebuild and each row grid in Row. An assigned definition keeps a SharedSizeGroup that reads back correctly and shares nothing, so every column sized alone while looking, from outside, exactly like a column that was sharing. Measured on a three-column table: the Auto column's heading started at x=416.0 and its cells at x=422.0. Six pixels, every row, for the life of the control. Star and absolute columns resolve to the same number in both grids without needing to share anything, so they lined up regardless - which is why this shipped and why it stayed. Fixed upstream by AvaloniaUI/Avalonia#21848, "register assigned definition collections with their shared size group", merged 2026-07-26 - after 12.1.0 was released on 2026-07-09. Populating works on 12.1.0 as it stands, so this costs no version bump and stays correct when the upstream fix does arrive. The guard is the part worth reading. It asserted that the SharedSizeGroup NAMES matched between the header grid and a row grid and that none was empty. Both were true the entire time nothing was sharing, so it passed every day of a defect it was written to catch. It had a second hole and that one is more instructive: the comment it was guarding says AUTO IS ACCEPTED AND MADE TO WORK, and no test in the file had ever used an Auto column. The one feature the comment claimed was the one feature the data could not exercise. It is replaced by an assertion about where the text actually lands - for every column, the heading's x equals its cell's x, through an Auto column whose heading is deliberately wider than its cells. Made to fail on purpose per §22.5: reverting Define to an assignment reports "Column 1 (classification) heading starts at x=356.0 but its cell starts at x=422.0", a 66 pixel gap naming the site to fix. The name assertion survives as a smaller claim, because it localizes a failure the positional one cannot explain. Two further guards. A 10,000 row table realizes 10 rows and 30 cells, which pins the assumption that row virtualization comes free from the ListBox and that cell virtualization follows from rows - a change of items panel would otherwise turn that into 10,000 grids with nothing to announce it but a slow window. And an upstream canary that fails the day a version carrying #21848 is taken, which is the notice that Define's comment has become history rather than a live hazard. Ui.Cols and Ui.Rows assign the same way and are deliberately left alone here. Their definitions parse from a comma-separated string that has no syntax for a SharedSizeGroup, so nothing there is trying to share and nothing is broken; it is a trap rather than a defect, and it gets its own change. Cites §27 rather than the correction subsection it wants, because that subsection is not written yet and CitationTests fails the build for a § that does not resolve.
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What does the pull request do?
Makes
Gridjoin a definition collection to the parent tree when the collection is assigned, rather than only when definitions are added to an already-parented collection. Without this, definitions supplied through theColumnDefinitions/RowDefinitionssetter are silently absent from theirSharedSizeGroup.Found while addressing review feedback on #21837.
What is the current behavior?
DefinitionList.SetParentassigns each definition'sParentbut never callsOnEnterParentTree, which only runs from the collection-changed handler. Definitions already present when the grid claims the collection therefore never join the parent tree.Assigning
Parentis not sufficient.OnEnterParentTreealso setsInheritanceParent, and a definition cannot read the inheritedPrivateSharedSizeScopethat registers it with its group until that link exists. The failure is silent: the grid lays out correctly in every other respect, and only the group membership is missing.This affects every route through the setter:
as well as
ColumnDefinitions="Auto,*"and aColumnDefinitionssupplied as a shared resource.The reverse case leaks. When a grid swaps in a new collection, the outgoing definitions keep their registration. They are no longer reachable from the grid, so nothing resets their measured minimum, and they pin the group at whatever size they last contributed — a shared column stays wide after the definitions that made it wide are gone.
What is the updated/expected behavior with this PR?
Definitions assigned as a collection register with their shared size group, and definitions replaced by a new collection unregister from it.
Validation on macOS ARM64 with .NET 10:
Shared_Size_Group_Is_Registered_For_Definitions_Assigned_As_A_CollectionandReplacing_Definition_Collection_Releases_Its_Shared_Size_Group. Each fails against unmodified sources on its own defect (0 instead of 50; 50 instead of 0) and passes with the fix.GridTests: 95 passed, 0 failed.Avalonia.Controls.UnitTests: 3,651 total, 3,650 passed, 1 pre-existing skip, 0 failed.Avalonia.Markup.Xaml.UnitTests: 591 total, 588 passed, 3 skipped, 0 failed.Avalonia.Base.UnitTests: 2,994 total, 2,982 passed, 12 skipped, 0 failed.Avalonia.Markup.UnitTests: 252 passed, 0 failed.How was the solution implemented (if it's not obvious)?
SetParentnow exits the old parent tree and enters the new one, so a definition's tree membership follows the collection however it was populated. It returns early when the parent is unchanged, which keeps theGridsetter's unconditionalParent = thisidempotent.Gridreleases the previous collection before adopting a new one. That release is what unregisters the outgoing definitions, so it depends on theSetParentchange.The commits follow the repository's bug-fix convention: the first adds the failing behavioral tests, the second contains the fix.
Checklist
Breaking changes
None. Both changes affect internal parent-tree bookkeeping only.
Obsoletions / Deprecations
None.
Fixed issues
None filed for this specific path. It may be the underlying cause of some reports of
SharedSizeGroupnot applying, but I have not verified any individual issue against it.