[Problem/Bug]: Runtime 157 suppresses <input type="file"> in Outlook add-in task panes ("top-level window size is smaller than the minimum required size (400x300)")

Since WebView2 Runtime 157.0.4341.42, clicking an element in an Office add-in task pane in Outlook desktop on Windows does nothing. No file chooser opens, no error is shown to the user and no change or cancel event fires. The same add-in works in Outlook on the web, and it works in Outlook desktop on Runtime 156.0.4314.58.

Our users started reporting this around Nov 5–6, 2026, as Runtime 157 reached their machines.

The task pane's DevTools console logs this warning on every click:

File chooser dialog was suppressed because the top-level window size is smaller than the minimum required size (400x300).
The warning comes from Chromium's chrome/browser/file_select_helper.cc. The new kMinFileChooserWindowSize (400×300) check is behind features::kFileChooserMinWindowSize, which is FEATURE_ENABLED_BY_DEFAULT (crbug.com/479258455). The same file also cancels an open chooser when the window is resized below that size.

In an Office task pane, the WebView2 that hosts the add-in is apparently treated as the "top-level window". A task pane is normally narrower than 400 px, so the chooser is always suppressed there. The protection against occlusion makes sense for small pop-up browser windows. But an embedded WebView2 sits inside a large host window (here the Outlook main window), so the check blocks a basic web platform feature for many WebView2 hosts.

Importance
Blocking. File selection is a core feature of our add-ins, and there is no workaround in the page itself: the input element is the standard way to pick a file.

Runtime Channel
Stable release (WebView2 Runtime)

Runtime Version
157.0.4341.42 (fails). Last working version: 156.0.4314.58.

SDK Version
N/A. The host is Microsoft Outlook (Office add-in runtime).

Framework
Other (Office Add-ins / Outlook)

Operating System
Windows 11

OS Version
Repro steps
Environment: Classic Outlook for Microsoft 365, Version 2610 (Build 16.0.20527.20088), 64-bit.

Sideload an Outlook add-in whose task pane contains only .
Open the task pane at its default width.
Click the file input. Nothing happens, and the console shows the warning above.
Point Outlook at Fixed Version Runtime 156.0.4314.58 through the BrowserExecutableFolder policy (value name OUTLOOK.EXE), then restart Outlook. Clicking the file input now opens the file chooser.
Point it at Fixed Version Runtime 157.0.4341.42. The chooser is suppressed again.
We used the policy only to compare versions. On affected user machines the Evergreen Runtime updated to 157 by itself.

Repros in Edge Browser
No. The same add-in works in Outlook on the web in Edge, where the top-level window is the browser window.

Regression
Regression in newer Runtime. Last working version: 156.0.4314.58.

Additional context
Developers can't work around this. Outlook sets the WebView2 browser arguments for add-ins itself (see #5770), so add-ins can't pass --disable-features=FileChooserMinWindowSize.
Request: turn off FileChooserMinWindowSize for WebView2, or measure the host's real top-level window (for example the window that contains the WebView2's parent HWND) instead of the WebView2's own size.

Currently the only workaround found is to increase the add-in pane to larger than 400px.