Feature/drag selection rectangle

[godlytalias]

Add Drag Selection Rectangle ( #334 )

Adds a semi-transparent rectangle overlay during cell/row drag selection, providing visual feedback similar to File Explorer's drag-select experience.

DragSelection.mp4

Feature Overview

• Visual drag rectangle — A semi-transparent accent-colored rectangle follows the mouse from the drag start point to the current pointer position, clamped to the viewport. The rectangle extends naturally when content scrolls (start point adjusts for scroll offset).
• Auto-scroll — When the pointer is near the top/bottom/left/right edges during drag, the view auto-scrolls with speed proportional to edge proximity. Vertical scroll uses a DispatcherTimer (16ms/~60fps) with ScrollViewer.ChangeView; horizontal scroll uses the existing HorizontalOffset DP pattern.
• Scroll-sync — Selection updates for newly visible rows during auto-scroll or mouse-wheel scroll via ScrollViewer.ViewChanged handler with hit-test at the clamped pointer position.
• ShowDragRectangle property — New bool dependency property (default: true) to enable/disable the rectangle. Setting to false mid-drag correctly tears down the rectangle. The underlying drag selection behavior (cell/row selection via pointer movement) is unaffected by this property.

Files Changed

File	Change
src/TableView.Properties.cs	ShowDragRectangle DP + property changed callback
src/TableView.cs	Drag rectangle fields, StartDragRectangle, UpdateDragRectangleVisual, PositionDragRectangle, auto-scroll timer, ViewChanged handler, FindCellAtCanvasPoint, EndDragRectangle cleanup, OnCurrentCellChanged drag optimization, OnCellSelectionChanged coalescing, OnUnloaded cleanup
src/TableViewCell.cs	OnPointerPressed starts drag, OnPointerReleased ends drag, OnPointerCaptureLost safety net, OnManipulationDelta updates rectangle visual, TransformPointToCanvas helper, ApplyCurrentCellState(skipFocus)
src/Themes/Resources.xaml	Theme resources for Light, Dark, HighContrast (fill, stroke, corner radius)
src/Themes/TableView.xaml	Canvas + Border overlay in control template (Grid.Row="1", IsHitTestVisible="False")
samples/.../SelectionPage.xaml	ShowDragRectangle ToggleSwitch + code substitution
tests/DragSelectionRectangleTests.cs	16 unit tests covering property defaults, drag lifecycle, selection mode guards, idempotency

Lifecycle

PointerPressed → CapturePointer + StartDragRectangle (hook ViewChanged, init state)
ManipulationDelta → UpdateDragRectangleVisual (position rectangle + auto-scroll)
                  → FindCell + MakeSelection (same path as non-rectangle)
ViewChanged → PositionDragRectangle (adjust for scroll) + FindCellAtCanvasPoint + MakeSelection
PointerReleased → EndDragRectangle (stop timer, unhook events, restore focus/scroll)
PointerCaptureLost → EndDragRectangle (safety net)

[w-ahmad]

Thanks for the PR @godlytalias, it looks great. Quick question: can we enable auto-scroll even when ShowDragRectangle is set to false? Currently the selection stops at the last visible cell without the drag rectangle.

[godlytalias]

Thanks for the PR @godlytalias, it looks great. Quick question: can we enable auto-scroll even when ShowDragRectangle is set to false? Currently the selection stops at the last visible cell without the drag rectangle.

@w-ahmad Sounds good, I guess we should then expose a property for devs to toggle it and have to keep it disabled by default for not breaking current behavior? We can enable the behavior only when DragSelectionRectangle being enabled? Or do you have different opinion?

[w-ahmad]

@w-ahmad Sounds good, I guess we should then expose a property for devs to toggle it and have to keep it disabled by default for not breaking current behavior? We can enable the behavior only when DragSelectionRectangle being enabled? Or do you have different opinion?

I think the auto-scroll must be there even when the DragSelectionRectangle is not visible and there is no need to add property to turn it on or off. This is because most DataGrid controls (tried on WPF and Synfusion) and even Excel offers it by default. The DragSelectionRectangle is a new feature for TableView, for which you’ve already added a flag property to turn it on or off.

[godlytalias]

@w-ahmad Sounds good, I guess we should then expose a property for devs to toggle it and have to keep it disabled by default for not breaking current behavior? We can enable the behavior only when DragSelectionRectangle being enabled? Or do you have different opinion?

I think the auto-scroll must be there even when the DragSelectionRectangle is not visible and there is no need to add property to turn it on or off. This is because most DataGrid controls (tried on WPF and Synfusion) and even Excel offers it by default. The DragSelectionRectangle is a new feature for TableView, for which you’ve already added a flag property to turn it on or off.

Okay sure, I will keep the autoscroll turned on by default then. Thanks

[w-ahmad]

Hey @godlytalias, just checking in on this PR. It looks quite close to being ready, and I'd love to include it in the upcoming v1.5 preview release.

Let me know if you're still planning to work on the remaining review items or if there's anything I can help with. Thanks!

[godlytalias]

Hey @godlytalias, just checking in on this PR. It looks quite close to being ready, and I'd love to include it in the upcoming v1.5 preview release.

Let me know if you're still planning to work on the remaining review items or if there's anything I can help with. Thanks!

@w-ahmad I think the only pending item is to convert the internal fields as internal properties. I will update that. However I saw some discussions around this in #346, Whether some conclusion is made regarding it?

[w-ahmad]

@godlytalias, That discussion seems aimed at making the cells able to click thru so users can interact directly with the rows. IMO, let's stay focused on completing and merging this PR, and we can come to that feature later.

[w-ahmad]

Looks good to me!
thank for your valuable contribution @godlytalias.