DataGrid: icon-only HeaderTemplate leaves the column unlabelled for screen readers

[mathewtaylor]

Follow-up from #423, which added HeaderTemplate to BbDataGridPropertyColumn. Pre-existing gap, not introduced by that change — but #423 makes it much easier to hit.

Problem

BbDataGridHeaderCell sets aria-sort, but the <th> carries no aria-label. Its accessible name therefore comes from its own text content. Title is used for the column chooser and the column menu's aria-labels, but it is not a fallback for the header cell itself.

So a header rendered from a template with no text — the icon-only case that HeaderTemplate exists to enable — produces an unlabelled column for screen reader users:

<BbDataGridPropertyColumn Property="x => x.Email" Title="Email">
    <HeaderTemplate>
        <LucideIcon Name="mail" Size="16" />
    </HeaderTemplate>
</BbDataGridPropertyColumn>

The column announces as empty, despite Title="Email" being right there and already being used elsewhere for exactly this purpose.

Current workaround

Consumers must add screen-reader-only text themselves, which is what the demo and the HeaderTemplate docs currently tell them to do:

<HeaderTemplate>
    <LucideIcon Name="mail" Size="16" />
    <span class="sr-only">Email</span>
</HeaderTemplate>

That works, but it puts the burden on the consumer for something the component already knows, and it fails silently when forgotten — nothing in the UI hints that the column is unlabelled.

Proposal

Have the header cell fall back to aria-label="@Title" when a HeaderTemplate is supplied (or, more simply, whenever Title is set and would otherwise not be the cell's text). The information is already there; it just is not reaching the element.

Notes

• Touches Primitives (BbDataGridHeaderCell), which is why it was kept out of Enhancement: DataGrid PropertyColumn HeaderTemplate #423.
• Worth checking the same question for BbDataGridTemplateColumn.HeaderTemplate, which has the identical shape and the identical exposure.
• Care needed not to double-announce when the template does contain text — an aria-label overrides the content, so applying it unconditionally would silence a perfectly good text header and could contradict it. Applying it only when Title is set is probably right, but the behaviour when both Title and template text exist should be decided deliberately.

[mathewtaylor]

Fixed in #445, merged to develop as 87ed018.

While a HeaderTemplate is in play and Title is non-empty, the header cell is now labelled from the title, so an icon-only header is announced correctly with nothing extra from the consumer. Plain text headers and templates without a Title are left untouched, so no existing markup changes announcement. The Primitives-layer BbDataGridHeaderCell also gains an AriaLabel parameter for grids assembled directly from primitives.

The previously-documented sr-only workaround degrades gracefully rather than double-announcing — existing templates that carry screen-reader text are announced once, by title, so there's nothing to unpick.