Unwanted cell selection in DataGridView when closing ContexMenuStrip

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Summary

An issue occurred in a WinForms application where the DataGridView (Dgv_Trovati) would unexpectedly select cells under the mouse cursor after a ContextMenuStrip was dismissed without selecting an item. The root cause was the interaction between the ContextMenuStrip closing event, the Control.MouseCapture property, and the DataGridView‘s internal mouse handling. The user provided a workaround using a flag (FattoClic) and manual selection logic in the CellMouseDown event to suppress the unintended selection behavior. The permanent architectural fix involves managing the ContextMenuStrip‘s AutoClose property or separating the context menu logic from the grid’s native selection mechanisms.

Root Cause

The behavior is a known interaction quirk in Windows Forms controls regarding mouse capture and message processing.

  • Native Mouse Capture Behavior: When a ContextMenuStrip is displayed, it captures the mouse. When the user clicks outside the menu (on the grid cell) to dismiss it, the capture is released.
  • Race Condition in DataGridView: The DataGridView tracks mouse movements to provide hover effects and selection highlighting. If the user moves the mouse immediately after clicking (dismissing the menu), the grid receives MouseMove events before it has fully processed the MouseUp event associated with the click.
  • Missing Hit-Test Validation: Because the grid perceives the mouse button as having been released (or validly pressed) in the context of a selection operation, and the mouse is moving over cells, the DataGridView interprets this as the user intending to perform a box selection or cell selection traversal.
  • The “Ghost” Selection: The grid begins selecting the cells the mouse passes over because it believes the user is dragging to select, even though the initial click was intended only to dismiss the menu.

Why This Happens in Real Systems

This is a side effect of UI State Management and Event Latency.

  • Focus and Activation: WinForms controls rely heavily on Windows Messages (WM_LBUTTONDOWN, WM_MOUSEMOVE, WM_MOUSEUP). When a modal or popup control (like a ContextMenuStrip) is active, the parent control (DataGridView) is in a “suppressed” input state.
  • State Desynchronization: Upon dismissal of the popup, the parent control does not always correctly reset its internal “drag selection” state immediately. If the MouseMove event fires before the ContextMenuStrip‘s Closing event completes or the DataGridView processes the MouseUp, the grid assumes the drag operation has started.

Real-World Impact

  • User Experience (UX) Destruction: Users hate it. It looks like a bug, breaks their workflow, and forces them to click randomly to clear the “sticky” selection.
  • Data Integrity Risk: If the user doesn’t notice the extra selection and proceeds to perform an action (e.g., “Delete Selected Rows”), they might accidentally modify or delete data they never intended to touch.
  • Accessibility Issues: Users with motor impairments who move the mouse slowly after clicking are disproportionately affected by this selection traversal.

Example or Code

The user provided a workaround using a flag FattoClic (DidClick) to prevent the CellMouseDown logic from running twice and to manually handle selection.

Private Sub CMStrip_Cerca_Closing(sender As Object, e As ToolStripDropDownClosingEventArgs) Handles CMStrip_Cerca.Closing
    Try
        Select Case e.CloseReason
            Case ToolStripDropDownCloseReason.ItemClicked, ToolStripDropDownCloseReason.AppClicked, ToolStripDropDownCloseReason.CloseCalled
                FattoClic = True
            Case Else
        End Select
    Catch
        FattoClic = True
    End Try
End Sub

Private Sub Dgv_Trovati_CellMouseDown(sender As Object, e As DataGridViewCellMouseEventArgs) Handles Dgv_Trovati.CellMouseDown
    If e.RowIndex = -1 OrElse e.ColumnIndex = -1 OrElse e.Clicks  1 Then Return

    If FattoClic AndAlso Not e.Button = MouseButtons.Right Then
        FattoClic = False
        If Me.Dgv_Trovati.SelectionMode = DataGridViewSelectionMode.RowHeaderSelect AndAlso Me.Dgv_Trovati.CurrentCell IsNot Nothing Then
            Me.Dgv_Trovati.Item(Me.Dgv_Trovati.CurrentCell.ColumnIndex, Me.Dgv_Trovati.CurrentCell.RowIndex).Selected = True
        End If
        Return
    End If

    Dim SelezionaInteraRiga As Boolean = Me.Dgv_Trovati.SelectionMode = DataGridViewSelectionMode.FullRowSelect

    If e.Button = MouseButtons.Right Then
        With Me.Dgv_Trovati
            If .SelectedCells.Count = 1 Then
                .Item(e.ColumnIndex, e.RowIndex).Selected = True
                .CurrentCell = .Item(e.ColumnIndex, e.RowIndex)
            ElseIf SelezionaInteraRiga Then
                .Rows(e.RowIndex).Selected = True
                .CurrentCell = .Item(e.ColumnIndex, e.RowIndex)
            End If
        End With
    ElseIf e.Button = MouseButtons.Left AndAlso Not KeysControlOrShiftTrue Then
        With Me.Dgv_Trovati
            .Item(e.ColumnIndex, e.RowIndex).Selected = True
            .CurrentCell = .Item(e.ColumnIndex, e.RowIndex)
            If SelezionaInteraRiga Then
                .Rows(e.RowIndex).Selected = True
                .CurrentCell = .Item(e.ColumnIndex, e.RowIndex)
            End If
        End With
    End If
End Sub

How Senior Engineers Fix It

Senior engineers avoid hacking CellMouseDown to fight the framework. Instead, they correct the state management or the UI architecture.

1. The “AutoClose” Solution (Recommended):
Stop the ContextMenuStrip from closing when the user clicks inside the DataGridView. This decouples the grid interaction from the menu dismissal logic.

Private Sub Dgv_Trovati_MouseDown(sender As Object, e As MouseEventArgs) Handles Dgv_Trovati.MouseDown
    If e.Button = MouseButtons.Right Then
        ' Get the hit test info
        Dim hit = Dgv_Trovati.HitTest(e.X, e.Y)

        If hit.Type = DataGridViewHitTestType.Cell OrElse hit.Type = DataGridViewHitTestType.RowHeader Then
            ' Select the row/cell explicitly if needed
            Dgv_Trovati.Rows(hit.RowIndex).Selected = True

            ' Show the menu
            CMStrip_Cerca.Show(Dgv_Trovati, e.Location)

            ' CRITICAL FIX: Prevent the menu from auto-closing immediately 
            ' when the mouse clicks the grid to open it.
            CMStrip_Cerca.AutoClose = False
        End If
    End If
End Sub

Private Sub CMStrip_Cerca_Closing(sender As Object, e As ToolStripDropDownClosingEventArgs) Handles CMStrip_Cerca.Closing
    ' Only allow closing if it's a valid reason, 
    ' ignoring the click that opened it or clicks inside the grid
    If e.CloseReason = ToolStripDropDownCloseReason.AppClicked Then
        e.Cancel = True
        ' Re-enable AutoClose for subsequent interactions
        CMStrip_Cerca.AutoClose = True
    End If
End Sub

2. The “Standard” Solution (Restoring Native Behavior):
Remove the custom CellMouseDown override entirely. If the goal is simply to select a row on right-click before showing the menu, use the MouseDown event strictly to trigger selection and the menu, and let the ContextMenuStrip handle its own dismissal. Do not manually set Selected = True inside the CellMouseDown event unless absolutely necessary, as it fights the DataGridView‘s native selection manager.

Why Juniors Miss It

  • Symptom vs. Cause: Juniors see “cells are selecting” and immediately write code to “unselect” or “block selection.” They don’t realize the selection is happening because the grid is confused about the Drag Operation state.
  • Over-reliance on Event Handlers: They try to fix everything inside CellMouseDown or MouseMove without considering the properties of the parent controls (ContextMenuStrip properties).
  • Lack of Win32 Awareness: They don’t understand that the ContextMenuStrip is a separate window handle that manipulates the message pump, affecting how messages reach the DataGridView (which is essentially a wrapper around the Win32 SysListView32 or custom draw window).
  • The “Band-Aid” Mentality: The user’s provided solution works, but it requires maintaining a global state flag (FattoClic) and complex logic to manage selection modes. Juniors often accept this as “working code” rather than seeking the clean architectural solution.