Add testing documentation for PDF fullscreen toggle fix

Co-authored-by: yeelam-gordon <73506701+yeelam-gordon@users.noreply.github.com>

TESTING_NOTES.md

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+# Testing Notes for PDF Fullscreen Toggle Fix
+
+## Issue
+[Peek] How do I un-fullscreen a PDF? (#Issue number to be filled)
+
+## Summary of Changes
+Fixed the PDF fullscreen toggle functionality in Peek. When viewing PDFs in Peek, clicking the fullscreen button now properly toggles between fullscreen and windowed modes.
+
+## Files Changed
+1. `src/modules/peek/Peek.FilePreviewer/Controls/BrowserControl.xaml.cs`
+   - Added `FullScreenChangedHandler` delegate and `FullScreenChanged` event
+   - Subscribed to `CoreWebView2.ContainsFullScreenElementChanged` event
+   - Added event handler `CoreWebView2_ContainsFullScreenElementChanged`
+
+2. `src/modules/peek/Peek.FilePreviewer/FilePreview.xaml`
+   - Wired up `FullScreenChanged` event handler to BrowserControl
+
+3. `src/modules/peek/Peek.FilePreviewer/FilePreview.xaml.cs`
+   - Added `FullScreenChanged` event
+   - Added `BrowserPreview_FullScreenChanged` event handler to propagate state
+
+4. `src/modules/peek/Peek.UI/PeekXAML/MainWindow.xaml`
+   - Wired up `FullScreenChanged` event handler to FilePreview
+
+5. `src/modules/peek/Peek.UI/PeekXAML/MainWindow.xaml.cs`
+   - Added `FilePreviewer_FullScreenChanged` event handler
+   - Toggles between `AppWindowPresenterKind.FullScreen` and `AppWindowPresenterKind.Default`
+
+## Manual Testing Steps
+
+### Prerequisites
+1. Build PowerToys with the changes
+2. Have a PDF file ready for testing
+
+### Test Case 1: Enter and Exit Fullscreen
+1. Open File Explorer
+2. Select a PDF file
+3. Press `Ctrl+Space` to open Peek
+4. Verify PDF is displayed correctly
+5. **Click the fullscreen button in the PDF viewer** (usually in bottom-right corner)
+6. **EXPECTED**: Window enters fullscreen mode, titlebar disappears
+7. **Click the fullscreen button again**
+8. **EXPECTED**: Window exits fullscreen mode, titlebar reappears
+
+### Test Case 2: Escape Key Still Works
+1. Open a PDF in Peek
+2. Click the fullscreen button to enter fullscreen
+3. Press `Escape` key
+4. **EXPECTED**: Window exits fullscreen AND Peek closes (existing behavior)
+
+### Test Case 3: Multiple Toggle Cycles
+1. Open a PDF in Peek
+2. Click fullscreen button (enter fullscreen)
+3. Click fullscreen button (exit fullscreen)
+4. Click fullscreen button (enter fullscreen again)
+5. Click fullscreen button (exit fullscreen again)
+6. **EXPECTED**: All toggles work correctly without any stuck states
+
+### Test Case 4: Navigation Between Files
+1. Select multiple PDF files in File Explorer
+2. Open Peek with `Ctrl+Space`
+3. Enter fullscreen mode
+4. Use arrow keys to navigate to next/previous PDF
+5. **EXPECTED**: Fullscreen state persists across file navigation
+
+### Test Case 5: Non-PDF Files
+1. Open a non-PDF file (e.g., image, text file) in Peek
+2. **EXPECTED**: No fullscreen button appears, behavior unchanged
+
+## Expected Behavior
+
+### Before Fix
+- Clicking fullscreen button hides titlebar but doesn't properly put window in fullscreen
+- Clicking fullscreen button again does nothing (stuck in pseudo-fullscreen)
+- Only way to exit is pressing Escape (which closes Peek entirely)
+
+### After Fix
+- Clicking fullscreen button properly enters fullscreen mode
+- Titlebar automatically hides
+- Window fills entire screen
+- Clicking fullscreen button again exits fullscreen
+- Titlebar reappears
+- Window returns to previous size/position
+
+## Technical Details
+
+The fix leverages WebView2's `ContainsFullScreenElementChanged` event which fires when:
+1. User clicks the PDF viewer's fullscreen button (enters fullscreen)
+2. User clicks the fullscreen button again (exits fullscreen)
+
+The event chain is:
+1. `CoreWebView2.ContainsFullScreenElementChanged` fires in BrowserControl
+2. BrowserControl raises `FullScreenChanged` event
+3. FilePreview propagates the event to MainWindow
+4. MainWindow calls `AppWindow.SetPresenter()` with appropriate presenter kind
+
+## Verification
+After testing, verify:
+- [ ] PDF fullscreen button enters fullscreen mode
+- [ ] PDF fullscreen button exits fullscreen mode (toggle works)
+- [ ] Titlebar visibility toggles correctly
+- [ ] Window size changes correctly
+- [ ] Escape key still closes Peek when in fullscreen
+- [ ] Navigation between PDFs maintains fullscreen state
+- [ ] No regressions with other file types