Start-Sleep - unlike System.Threading.Thread.Sleep - in a WinForms DoEvents loop prevents reactivating a minimized window

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest released version
• Search the existing issues.
• Refer to the FAQ.
• Refer to Differences between Windows PowerShell 5.1 and PowerShell.

Steps to reproduce

Note:

• This is clearly not a common use case, but it would still be good to know the cause of the problem.
• WPF windows are equally affected.
• Using [System.Threading.Thread]::Sleep() in lieu of Start-Sleep makes the problem go away.
• See below for additional details regarding what messages are missed.

Repro steps:

• Run the following code, which shows a WinForms form non-modally and then uses a DoEvents() loop with periodic Start-Sleep calls to process GUI events until the form is closed:

Add-Type -AssemblyName System.Windows.Forms

# Create a sample form.
$form = [System.Windows.Forms.Form] @{
  Text = "Sample"; Size = [System.Drawing.Size]::new(600, 200); StartPosition = 'CenterScreen'
}

# Show the form *non-modally*
$null = $form.Show()

while ($form.Visible) {

  # Allow the form to process events.
  [System.Windows.Forms.Application]::DoEvents()

  # Sleep a little, to avoid a near-tight loop.
  # !! THIS CAUSES THE PROBLEM.
  # !! If you replace the Start-Sleep call with a call
  # !! to [System.Threading.Thread]::Sleep(), the problem goes away.
  Start-Sleep -Milliseconds 50

}

• Minimize the form.

• Hover over the PowerShell instance’s taskbar button and try to reactivate the form by clicking on its thumbnail.

Expected behavior

The form should be restored and activated.

Actual behavior

Nothing happens (while you hover, the form is previewed in its original position after a brief delay, but clicking the thumbnail is ineffective in restoring and activating it; clicking on the thumbnail’s close button does close the form).

Also, trying to Alt-Tab to the window doesn’t activate it either.

Error details

No response

Environment data

PowerShell 7.4.0-preview.3 on Windows 11 22H2

Visuals

No response

[fMichaleczek]

The Cmdlet Start-Sleep uses System.Threading.ManualResetEvent API instead of System.Threading.Thread.Sleep I can reproduce an issue between the method DoEvents and the ManualResetEvent class. It can also be resolved it by using a unique ManualResetEvent instance. (something that Start-Sleep is not doing I guess)

Not Working :

Add-Type -AssemblyName System.Windows.Forms
$form = [System.Windows.Forms.Form]@{
    Text = 'Sample'
    Size = [System.Drawing.Size]::new(600, 200)
    StartPosition = 'CenterScreen'
}
[void] $form.Show()
while ($form.Visible) {
    [void] [System.Windows.Forms.Application]::DoEvents()
    $waitHandle = [System.Threading.ManualResetEvent]::new(<# initialState: #> $false)
    [void] $waitHandle.WaitOne(<# millisecondsTimeout: #> 50, $true)
    [void] $waitHandle.Set()
    $waitHandle.Close()
}

Working :

Add-Type -AssemblyName System.Windows.Forms
$form = [System.Windows.Forms.Form]@{
    Text = 'Sample'
    Size = [System.Drawing.Size]::new(600, 200)
    StartPosition = 'CenterScreen'
}
[void] $form.Show()
$waitHandle = [System.Threading.ManualResetEvent]::new(<# initialState: #> $false)
while ($form.Visible) {
    [void] [System.Windows.Forms.Application]::DoEvents()
    [void] $waitHandle.WaitOne(<# millisecondsTimeout: #> 50, $true)
    [void] $waitHandle.Set()
} 
$waitHandle.Close()

Working :

Add-Type -AssemblyName System.Windows.Forms
$form = [System.Windows.Forms.Form]@{
    Text = 'Sample'
    Size = [System.Drawing.Size]::new(600, 200)
    StartPosition = 'CenterScreen'
}
[void] $form.Show()
while ($form.Visible) {
    [void] [System.Windows.Forms.Application]::DoEvents()
    if ($form.Focused) {
        $waitHandle = [System.Threading.ManualResetEvent]::new(<# initialState: #> $false)
        [void] $waitHandle.WaitOne(<# millisecondsTimeout: #> 50, $true)
        [void] $waitHandle.Set()
        $waitHandle.Close()
    }
}

Working :

Add-Type -AssemblyName System.Windows.Forms

$form = [System.Windows.Forms.Form]@{
    Text = 'Sample'
    Size = [System.Drawing.Size]::new(600, 200)
    StartPosition = 'CenterScreen'
}
[void] $form.Show()

while ($form.Visible) {
    [void] [System.Windows.Forms.Application]::DoEvents()
    $waitHandle = [System.Threading.ManualResetEvent]::new(<# initialState: #> $false)
    if ($form.WindowState -ne 'Minimized') {
        [void] $waitHandle.WaitOne(<# millisecondsTimeout: #> 50, $true)
    }
    [void] $waitHandle.Set()
    $waitHandle.Close()
}

[lippmaje]

I can confirm this behavior for Windows 7 SP1, Powershell 5.1, .NET 4.8 (cannot test on newer OS right now). With Start-Sleep, the dialog misses the (WM_SYSCOMMAND, SC_MINIMIZE=0xF020) message that is sent if the taskbar icon is clicked while the dialog is open - likewise it misses the (WM_SYSCOMMAND, SC_RESTORE=0xF120) when the dialog was minimized, leaving it pretty inoperable in the taskbar. This does not happen when waiting with [Threading.Thread]::Sleep(20).

I also want to note that this behavior is independent from running the Winforms event handler or a WPF pendant.