ci(ui-tests): take a screenshot when canceled

Sometimes the logs are empty and it is highly unclear what has happened.
In such a scenario, a picture is indeed worth more than a thousand
words.

Note that this commit is more complicated than anyone would like, for
two reasons:

- While PowerShell is the right tool for the job, a PowerShell step in
  GitHub Actions will pop up a Terminal window, _hiding_ what we want to
  screenshot. To work around that, I tried to run things in a Bash step.
  _Also_ opens a Terminal window! Node.js to the rescue.

- _Of course_ it is complicated to take a screenshot. The challenge is
  to figure out the dimensions of the screen, which should be as easy as
  looking at `[System.Windows.Forms.Screen]::PrimaryScreen`'s `Bounds`
  attribute.

  Easy peasy, right? No, it's not. Most machines nowadays have a
  _ridiculous_ resolution which is why most setups have a _zoom factor_.
  Getting to that factor should be trivial, by calling
  `GetDeviceCaps(hDC, LOGPIXELSX)`, but that's not working in modern
  Windows! There is a per-monitor display scaling ("DPI"). But even
  _that_ is hard to get at, calling `GetDpiForMonitor()` will still
  return 96 DPI (i.e. 100% zoom) because PowerShell is not marked as
  _Per-Monitor DPI Aware_. Since we do not want to write a manifest into
  the same directory as `powershell.exe` resides, we have to jump
  through yet another hoop to get that.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

.github/workflows/ui-tests.yml

@@ -94,6 +94,63 @@ jobs:
         if: cancelled()
         working-directory: ui-tests
         run: type bg-hook.log
+      - name: Take screenshot, if canceled
+        id: take-screenshot
+        if: cancelled() || failure()
+        shell: powershell
+        run: |
+          Add-Type -TypeDefinition @"
+          using System;
+          using System.Runtime.InteropServices;
+
+          public class DpiHelper {
+              [DllImport("user32.dll")]
+              public static extern bool SetProcessDpiAwarenessContext(IntPtr dpiContext);
+
+              [DllImport("Shcore.dll")]
+              public static extern int GetDpiForMonitor(IntPtr hmonitor, int dpiType, out uint dpiX, out uint dpiY);
+
+              [DllImport("User32.dll")]
+              public static extern IntPtr MonitorFromPoint(System.Drawing.Point pt, uint dwFlags);
+
+              [DllImport("user32.dll")]
+              public static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);
+
+              public static uint GetDPI() {
+                  // DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2 = -4
+                  SetProcessDpiAwarenessContext((IntPtr)(-4));
+
+                  uint dpiX, dpiY;
+                  IntPtr monitor = MonitorFromPoint(new System.Drawing.Point(0, 0), 2); // MONITOR_DEFAULTTONEAREST
+                  GetDpiForMonitor(monitor, 0, out dpiX, out dpiY); // MDT_EFFECTIVE_DPI
+                  return (dpiX + dpiY) / 2;
+              }
+          }
+          "@ -ReferencedAssemblies "System.Drawing.dll"
+
+          # First, minimize the Console window in which this script is running
+          $hwnd = (Get-Process -Id $PID).MainWindowHandle
+          $SW_MINIMIZE = 6
+
+          [DpiHelper]::ShowWindow($hwnd, $SW_MINIMIZE)
+
+          # Now, get the DPI
+          $dpi = [DpiHelper]::GetDPI()
+
+          # This function takes a screenshot and saves it as a PNG file
+          [Reflection.Assembly]::LoadWithPartialName("System.Drawing")
+          function screenshot([Drawing.Rectangle]$bounds, $path) {
+            $bmp = New-Object Drawing.Bitmap $bounds.width, $bounds.height
+            $graphics = [Drawing.Graphics]::FromImage($bmp)
+            $graphics.CopyFromScreen($bounds.Location, [Drawing.Point]::Empty, $bounds.size)
+            $bmp.Save($path)
+            $graphics.Dispose()
+            $bmp.Dispose()
+          }
+          Add-Type -AssemblyName System.Windows.Forms
+          $screen = [System.Windows.Forms.Screen]::PrimaryScreen
+          $bounds = [Drawing.Rectangle]::FromLTRB(0, 0, $screen.Bounds.Width * $dpi / 96, $screen.Bounds.Height * $dpi / 96)
+          screenshot $bounds "ui-tests/screenshot.png"
       - name: Upload test results
         if: always()
         uses: actions/upload-artifact@v4