The SelectFileDialogExtension class is used to select a file from an extension or app.  The dialog is associated with a unique id (SelectFileDialogExtension::RoutingID) [1], which is based on a WebContents [2].  This routing id is used to programmatically interact with the dialog through the fileManagerPrivate API (e.g., fileManagerPrivate.cancelDialog) [3].

The dialog itself is triggered through the SelectFileDialog::SelectFile flow from the fileBrowserHandlerInternal.selectFile extension API function. [4]  The routing ID that is associated with this dialog is determined by calling FindRuntimeContext() for the gfx::NativeWindow passed into SelectFileDialogExtension::SelectFileImpl() [5].  FindRuntimeContext() looks at the gfx::NativeWindow either the associated browser or the associated app window.  If it can't find either, it finds some other web contents to use (such as the last active browser's active tab).  The gfx::NativeWindow passed into this is from the browser retrieved by ChromeExtensionFunctionDetails::GetCurrentBrowser() for the fileBrowserHandlerInternal.selectFile API function [6].  This, in turn, searches a number of ambiguous web contents and browsers, also occasionally defaulting to the active browser (or even an inactive one. [7]

In order to retrieve this same web contents, the code in fileManagerPrivate uses the deprecated GetAssociatedWebContents() function. [8]  This function, too, is fairly non-deterministic in the web contents it returns.

So, we end up having a flow like the following:
1. fileBrowserHandlerInternal.selectFile calls SelectFileDialog::SelectFile with a window of a browser retrieved using ChromeExtensionFunctionDetails::GetCurrentBrowser().
2. The SelectFileDialogExtension::SelectFileImpl method associates this dialog with a WebContents retrieved from FindRuntimeContext().
3. fileManagerPrivate.cancelDialog retrieves the ID for a WebContents with ChromeExtensionFunctionDetails::GetAssociatedWebContents().

All three of these stages - using GetCurrentBrowser(), FindRuntimeContext(), and GetAssociatedWebContents() return unpredictable values, but it's required that they match.  This makes it fragile, and difficult to understand.  Instead, these should find a way of using a deterministic value associated with the caller.

I attempted to transition at least the third step to using GetSenderWebContents() (which is deterministic and predictable) in revision 1c397d3ddc70d0a4ffacb9bec37d447d8dd8ec9f, but this had to be reverted in eaffdd8c5fb9f81bc7956de38389aad8048bcfc8 because of crashes (https://crbug.com/842581).  At this point, I think this will take someone looking more holistically into this flow.

Since the fileManagerPrivate API is the last caller of GetAssociatedWebContentsDeprecated(), I'm planning on just inlining the method there to prevent any further use.

[1] https://chromium.googlesource.com/chromium/src/+/82a5f004e0e52897a6e8bac10490a14fc2845625/chrome/browser/ui/views/select_file_dialog_extension.h#38
[2] https://chromium.googlesource.com/chromium/src/+/82a5f004e0e52897a6e8bac10490a14fc2845625/chrome/browser/ui/views/select_file_dialog_extension.cc#373
[3] https://chromium.googlesource.com/chromium/src/+/82a5f004e0e52897a6e8bac10490a14fc2845625/chrome/browser/chromeos/extensions/file_manager/private_api_dialog.cc#32
[4] https://chromium.googlesource.com/chromium/src/+/82a5f004e0e52897a6e8bac10490a14fc2845625/chrome/browser/chromeos/extensions/file_manager/file_browser_handler_api.cc#218
[5] https://chromium.googlesource.com/chromium/src/+/82a5f004e0e52897a6e8bac10490a14fc2845625/chrome/browser/ui/views/select_file_dialog_extension.cc#120
[6] https://chromium.googlesource.com/chromium/src/+/01ce72edb648ed9295f00c97afbe5eda1619c422/chrome/browser/chromeos/extensions/file_manager/file_browser_handler_api.cc#311
[7] https://chromium.googlesource.com/chromium/src/+/01ce72edb648ed9295f00c97afbe5eda1619c422/chrome/browser/extensions/chrome_extension_function_details.cc#35
[8] https://chromium.googlesource.com/chromium/src/+/82a5f004e0e52897a6e8bac10490a14fc2845625/chrome/browser/chromeos/extensions/file_manager/private_api_dialog.cc#25