HTML <select> dropdowns do not open in CefSharp.Wpf OSR until the window is resized or the page is reloaded

[onlylau]

I am experiencing an issue with HTML <select> elements in a WPF application using CefSharp.Wpf with off-screen rendering.

After navigating to a page, clicking a <select> element does not display the dropdown popup. The rest of the page remains responsive and clickable.

The issue is not limited to one specific page. It occurs across different pages of the website.

The dropdown starts working immediately after either:

• Minimizing and restoring the application window
• Resizing the application window
• Reloading the page

However, after navigating to another page, the issue usually returns.

Environment

• Application type: WPF
• CefSharp package: CefSharp.Wpf
• CefSharp version: 149.0.60
• Rendering mode: OSR / windowless rendering
• Platform: Windows x64
• .NET application written in C#
• The issue appeared after upgrading from CefSharp 147 to CefSharp 149, although I am still verifying whether it is a regression

Steps to reproduce

1. Start the WPF application.
2. Initialize the ChromiumWebBrowser.
3. Navigate to a website containing standard HTML <select> elements.
4. Navigate to another page inside the website.
5. Click a <select> element.

Actual result

The click appears to be received, but the dropdown popup is not visible.

Workarounds that immediately restore the dropdown

• Resize the WPF window by a few pixels.
• Minimize and restore the WPF window.
• Reload the page.

After a resize, the same <select> works correctly without reloading the document.

Navigating to another page may cause the issue to return.

Expected result

The <select> dropdown popup should appear normally after every page navigation without requiring a window resize or page reload.

Important observation

The fact that resizing or minimizing/restoring the WPF window fixes the problem without reloading the page suggests that the issue may be related to OSR rendering, popup surface synchronization, browser size notification, or popup geometry.

In CefSharp WPF OSR, the <select> dropdown appears to be rendered through the separate popup surface, including:

• OnPopupShow
• OnPopupSize
• OnPaint with isPopup = true
• PART_popupImage

The main browser surface continues to render correctly, but the popup surface may remain invalid or unsynchronized after navigation.

A possible sequence is:

Page navigation
→ new document/frame finishes loading
→ main OSR view renders correctly
→ popup OSR surface remains invalid or unsynchronized
→ clicking <select> does not show the popup
→ a real window resize triggers layout/WasResized/invalidation
→ the popup starts working

Browser visibility handling

The application contains a custom ChromiumWebBrowser subclass with an override similar to this:

protected override void OnBrowserWasHidden(bool hidden)
{
    var browserHost = GetBrowserHost();

    if (browserHost != null)
    {
        browserHost.WasHidden(false);
        browserHost.WasResized();
        browserHost.Invalidate(PaintElementType.View);
    }
}

This was added to keep the browser active while the WPF control or application is not visible.

I am currently investigating whether forcing WasHidden(false) may leave the internal CEF state different from the actual WPF visibility state.

The issue also appears to be related to page navigation, not only to hiding and showing the browser.

Cef command-line arguments

The application currently uses some non-default CEF command-line arguments:

disable-gpu
disable-gpu-compositing
disable-features=RenderDelay,LazyFrameLoading,PaintHolding
enable-experimental-web-platform-features
disable-back-forward-cache

I understand that these flags may affect OSR rendering or popup composition. I am preparing isolated tests without these flags.

The most suspicious flags currently are:

disable-gpu
disable-gpu-compositing
enable-experimental-web-platform-features

Tests performed

Test 1 — Reload

Reloading the page makes the <select> dropdown work.

Test 2 — Minimize and restore

Minimizing and restoring the WPF window makes the dropdown work without reloading the page.

Test 3 — Small window resize

Changing the window size by a few pixels also makes the dropdown work.

Test 4 — Navigate to another page

After navigating to another page, the dropdown may stop appearing again.

This makes a JavaScript preventDefault() issue less likely, because resizing the native WPF window fixes the existing document without changing the JavaScript or DOM.

Suspected areas

The issue may involve one or more of the following:

• OSR popup surface not being recreated after navigation
• Missing or delayed WasResized() after page load
• Popup geometry being calculated before WPF layout is stable
• PART_popupImage not being updated or displayed
• Software rendering caused by disable-gpu and disable-gpu-compositing
• Incorrect WasHidden state
• Regression between CefSharp 147 and 149
• DPI or WPF layout synchronization
• ResizeHackEnabled

Questions

1. Is this a known issue in CefSharp 149 or the matching CEF/Chromium version?

2. Are there known regressions involving OSR <select> popups after page navigation?

3. Is ResizeHackEnabled expected to help with this type of issue?

4. Is it safe or recommended to call WasResized() and Invalidate() after the main frame finishes loading?

5. Could forcing WasHidden(false) while the WPF browser is actually hidden cause popup rendering problems?

6. Are disable-gpu and disable-gpu-compositing known to affect popup widgets in CefSharp WPF OSR?

7. Which callbacks would be most useful to log:

   • OnPopupShow
   • OnPopupSize
   • OnPaint(isPopup: true)
   • OnBrowserWasHidden
   • OnRenderSizeChanged

[amaitland]

Download https://cef-builds.spotifycdn.com/cef_binary_149.0.6%2Bg0d0eeb6%2Bchromium-149.0.7827.201_windows64_client.tar.bz2

Test with

cefclient.exe --multi-threaded-message-loop --no-sandbox --off-screen-rendering-enabled --enable-gpu