Tabular: add opt-in column-header tooltips to TableView

[Hitesh Kumar (hiteshkrmsft)]

Adds TableViewColumn.HeaderToolTip, the header-side counterpart to the per-cell CellToolTipBinding that shipped in #11669.

<tabular:TableViewTextColumn Header="Notes"
                             Binding="{Binding Notes}"
                             CellToolTipBinding="{Binding Notes}"
                             HeaderToolTip="Free-form notes captured at intake" />

A dependency property typed Object, not a Binding: a header is not bound against a row, so there is nothing to defer. x:Bind and StaticResource both still work against it.

Design

The value is the tooltip's content, not a ToolTip; the control owns the ToolTip and its placement (PlacementMode.Mouse), and never touches a tooltip the app set. That is the merged cell contract, and the ownership record, the content rules and the teardown are reused rather than duplicated — SetOwnedToolTip gained an opt-out for publishing AutomationProperties.HelpText, and nothing else changed in the cell path.

The tooltip is attached to the header cell root, so it covers the header content, its padding and its sort affordance.

Header cells are rebuilt wholesale rather than recycled, so there is no binding, no attached value and no refresh path: the content is read from the column when the header is built, and re-applied in place when the property changes. Re-applying rather than rebuilding keeps header focus and the sort affordance intact, and deliberately avoids OnColumnHeaderChanged, which recomputes the column's Auto width — a tooltip cannot change it. Owned tooltips are closed before RebuildHeaders discards the band, since clearing an open tooltip's element is the reentrant CPopup::RemoveChild shape.

Accessibility

String content is reported through TableViewColumnHeaderAutomationPeer::GetHelpTextCore, composed with the column's sort state through a localized format string so neither is dropped. It is suppressed when it merely repeats the header's own name, so the same text is never announced twice — the header-side equivalent of the suppression TableViewCellAutomationPeer already does, and equally gated on the ownership record, so text the app set is never dropped.

This is the one place the header genuinely differs from a cell. The header peer is virtual — owned by the TableView, not by the header element — so AutomationProperties.HelpText on that element would never reach a client, and GetHelpTextCore already reported the sort state. The peer therefore publishes the tooltip text itself and merges the two, rather than the attach path setting HelpText as it does for cells.

Non-string content is mouse-only, matching the cell rule and TabViewItem / NavigationViewItem.

Spec correction

The functional spec claimed column-header tooltips were deferred because the header band is IsHitTestVisible="False" and "a tooltip there could never open". That is wrong: the attribute is on the row's selection indicator, and the header band is hit-testable — which is what makes click-to-sort and resize work. Corrected in this PR.

Not in scope

Tooltips are still not gated on truncation. The header ContentPresenter has no TextTrimming, so header text currently clips without an ellipsis while its cells ellipsize correctly. Making headers trim and then auto-showing a tooltip only when trimmed — for headers and cells — is the natural follow-up, and is worth its own review: it changes the default for every table, and it would also close the non-string accessibility gap, since a truncation-driven tooltip is text by construction. Group-header tooltips remain deferred until grouping is enabled.

Validation

Built and verified locally:

• Microsoft.UI.Xaml.Controls.dll and Microsoft.UI.Xaml.Controls.Tabular.dll build and link clean (x64).
• The produced Microsoft.UI.Xaml.winmd carries TableViewColumn.HeaderToolTip with get_/put_ accessors plus HeaderToolTipProperty, read out of the metadata tables.
• XamlMetadataProviderGenerated.h registers the DP, so HeaderToolTip="…" resolves from markup.
• The regenerated CsWinRT projection exposes HeaderToolTip to C#, and the ToolTipsPage sample — extended here with header tooltips (text on Name and City, non-string content on Bio, and a tooltip on Score that repeats its own header text to exercise the suppression) plus a button that reads the header peers' help text — compiles against it.

The sample could not be run. TableViewSampleApp crashes at startup with 0xC000027B (stowed E_UNEXPECTED from Microsoft.UI.Xaml.dll) in this tree. That is pre-existing and not caused by this change: an A/B control built from main through the identical package pipeline, differing only in this commit, crashes identically. So the runtime behaviour — tooltip open, placement, teardown on rebuild, in-place re-apply, and the peer's help-text composition — is left to CI and to review.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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