Add built-in TreeView checkbox mode

[Copilot]

Fixes

TreeView multi-selection only rendered highlighted rows, requiring custom rendering/input code for checkbox-style trees. This adds native checkbox rendering and toggling for node-level checked state.

Proposed Changes/Todos

• Add TreeView<T>.CheckboxMode for built-in checkbox glyph rendering.
• Add checked-state API:
  • GetCheckedObjects()
  • GetCheckState(T?)
  • SetChecked(T?, CheckState)
  • CheckedChanged
• Support Space-to-toggle when checkbox mode is enabled.
• Support mouse toggling when clicking the checkbox glyph only.
• Render parent nodes as indeterminate when child check state is mixed.

tree.CheckboxMode = true;
tree.SetChecked(node, CheckState.Checked);

foreach (object checkedNode in tree.GetCheckedObjects ())
{
    // consume checked nodes
}

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

[tznind]

Does it work with treetable source though? The source for tableview that makes column 0 expandable tree?

[tig]

@tznind this is not really ready for review.

Does it work with treetable source though? The source for tableview that makes column 0 expandable tree?

Can you explain this more? I may be missing something that's already there. thanks!

[tznind]

@tznind this is not really ready for review.

Does it work with treetable source though? The source for tableview that makes column 0 expandable tree?

Can you explain this more? I may be missing something that's already there. thanks!

public class TreeTableSource : IEnumerableTableSource, IDisposable where T : class

It combines treeview with tableview. Giving you columns. Also you can wrap the source with the checkbox wrapper source that gaven you checkbox column at start.

But i believe that wrapper got replaced with direct checkbox support in table view at some point (if my memory serves)

[tig]

@tznind this is not really ready for review.

Does it work with treetable source though? The source for tableview that makes column 0 expandable tree?

Can you explain this more? I may be missing something that's already there. thanks!

public class TreeTableSource : IEnumerableTableSource, IDisposable where T : class

It combines treeview with tableview. Giving you columns. Also you can wrap the source with the checkbox wrapper source that gaven you checkbox column at start.

But i believe that wrapper got replaced with direct checkbox support in table view at some point (if my memory serves)

Good question. No, CheckboxMode is purely for standalone TreeView rendering; it adds the checkbox glyph in Branch.Draw(). When TreeView is used as a data model behind TreeTableSource, the checkbox glyphs aren't rendered in the table column (since GetColumnZeroRepresentationFromTree only pulls prefix + expand symbol + AspectGetter text).

For tree-table checkboxes, the existing CheckBoxTableSourceWrapper pattern still applies — wrap the TreeTableSource with it. The two mechanisms are independent.

I've documented this distinction in both treeview.md and tableview.md.

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[Copilot]

Pull request overview

Adds first-class checkbox support to TreeView<T>, including check-state APIs, checkbox rendering/toggling, UICatalog editor exposure, documentation, and tests.

Changes:

• Adds CheckboxMode, checked-state storage/API, CheckedChanged, checkbox glyph rendering, and keyboard/mouse toggling.
• Adds tests for checkbox rendering, propagation, tri-state behavior, and mouse interactions.
• Updates TreeView/TableView docs and UICatalog editor controls for the new mode.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
Terminal.Gui/Views/TreeView/TreeViewT.cs	Adds checkbox mode state, event, cleanup, and activation handling.
Terminal.Gui/Views/TreeView/TreeView.Navigation.cs	Adds check-state APIs and recursive check-state helpers.
Terminal.Gui/Views/TreeView/Branch.cs	Renders checkbox glyphs and updates sizing/hit testing.
Terminal.Gui/Views/TreeView/ITreeView.cs	Adds checkbox mode to the interface.
Terminal.Gui/Views/TreeView/CheckedChangedEventArgs.cs	Adds event args for check-state changes.
Examples/UICatalog/Scenarios/EditorsAndHelpers/TreeViewEditor.cs	Exposes checkbox mode in the TreeView editor scenario.
Tests/UnitTestsParallelizable/Views/TreeViewTests.cs	Adds checkbox mode behavioral and rendering coverage.
docfx/docs/treeview.md	Documents checkbox mode usage and semantics.
docfx/docs/tableview.md	Clarifies TreeTableSource checkbox behavior.

[BDisp]

Nice.