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

@@ -93,6 +93,75 @@ jobs:
         if: cancelled()
         working-directory: ui-tests
         run: type bg-hook.log
+      - name: Take screenshot, if canceled
+        id: take-screenshot
+        if: cancelled() || failure()
+        uses: actions/github-script@v7 # `pwsh` and `bash` open a Terminal window!
+        with:
+          script: |
+            const powershell = `
+            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);
+
+                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"
+
+            $dpi = [DpiHelper]::GetDPI()
+
+            [Reflection.Assembly]::LoadWithPartialName("System.Drawing")
+            function screenshot([Drawing.Rectangle]$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"
+            `
+
+            const { spawn } = require('child_process')
+
+            const child = spawn('powershell.exe', ['-Command', powershell], {
+              windowsHide: true,
+              shell: false
+            })
+
+            child.stdout.on('data', (data) => {
+              console.log(`Output: ${data}`)
+            })
+
+            child.stderr.on('data', (data) => {
+              console.error(`Error: ${data}`)
+            })
+
+            child.on('exit', (code) => {
+              console.log(`Script exited with code ${code}`)
+              process.exit(code)
+            })
       - name: Upload test results
         if: always()
         uses: actions/upload-artifact@v4