window.close() Restrictions

Sometimes, Web Developers are surprised to find that the window.close() API doesn’t always close the browser window.

When looking at the Developer Tools console, they’ll see a message like:

Scripts may close only the windows that were opened by them.

Why Do Browsers Limit close()?

Before we dive into what factors govern what happens when close() is called, it’s important to understand why there’s a restriction at all.

While sometimes explained away as nebulous “Security reasons!“, the core reason is more aligned with simple user-experience— users could lose important state in their browser window or “back/forward” stack (in IE, we called this the TravelLog) if a tab or window went away unexpectedly. The user might’ve been using a tab to explore a set of search results, and if one of those results could blow away both itself and the backstack with the search results page, this would be pretty annoying.

There’s a minor anti-abuse/security argument as well– if a browser tab could blow itself away freely, this might be useful as a part of scareware or other user-experience abuses.

What Does the Standard Say?

Here’s what the dom-window-close section of the HTML Standard has to say:

A browsing context is script-closable if it is an auxiliary browsing context that was created by a script (as opposed to by an action of the user), or if it is a top-level browsing context whose session history contains only one Document.

This seems simple enough, although the parts I’ve bolded hide a lot of complexity and nuance. (Most obviously, “what should we do if the script was run in response to an action of the user?“)

What Do Browsers Do?

Unfortunately for us, each browser has a different set of behaviors (explore with this test page), partly because most were implemented before the standard was written.

Internet Explorer

In Internet Explorer, a tab/window will be closed silently if it was created as a result of a JavaScript window.open() call. There’s no attempt to check whether the back/forward stack contains only one document: a tab with a large TravelLog will still close silently if it was opened by script. (IE also allows HTA documents to close themselves without restriction.)

In all other circumstances, the tab/window will not silently close: instead, the user is presented with a one of two modal dialogs, depending on whether the page represents the only tab in the browser window:

Chromium (Microsoft Edge / Google Chrome / etc)

As of Chromium 88, window.close() succeeds if the new window/tab has an opener or if the back/forward stack contains fewer than two entries.

As you can see, there are subtle differences here between what the spec requires and what the browser implements.

First, notice that I said “has an opener” rather than “was created by a script.” Recall that the opener property allows a popup window to refer to the tab that created it.

  • If the user creates a new tab by clicking the frame button, hitting Ctrl+T, Shift+Clicking a link, or launching a URL from the Shell, the resulting new tab doesn’t have an opener set.
  • In contrast, if the tab is opened via open() or hyperlink with a named target (not _blank), then by default, the opener property is set.
  • Any hyperlink may include rel=opener or rel=noopener to specify whether the new tab has an opener set.
  • A open() JavaScript call can specify noopener in its windowFeatures string to set the new tab’s opener to null.

As you can see from the above list, both normal link clicks and JavaScript open() invocations can result in tabs with or without the opener set. This can be very confusing: Shift+click on a link might result in a tab that cannot self-close, while Left-click on that same link results in a tab that can always close itself.

Secondly, notice I said “entries” rather than “Documents.” In most cases, these are equivalent, but they’re not exactly the same. Consider the case where the new tab navigates to a HTML document with a Table of Contents at the top. The user clicks on a ToC link to #Section3 and the browser dutifully scrolls down to that section. The back/forward stack now contains two entries, both pointing to the same document. Chromium blocks window.close(), but it shouldn’t. This longstanding shortcoming was made more visible in Chromium 88, which now gives links targeting _blank the noopener behavior by default.

crbug.com/1170131 tracks fixing this issue by counting the number of Documents in the back/forward stack, but it will be tricky to fix because presently the renderer process running JavaScript has access only to the count of entries in the back/forward stack, not their URLs.

Chromium: User-Experience

When Chrome blocks close(), it emits a notice to the console:

Scripts may close only the windows that were opened by them.

…but there’s no notice to the user, who may be left confused if they clicked on a “Close” button or link inside the page. Recently filed crbug.com/1170034 suggests introducing a dialog box like Internet Explorer’s. (As an aside, it also sets a new standard for bug filing by including comic-book style art showing unhappy users converting to happy users if the proposed feature is landed. :)

Chromium: Exotic Bug Trivia

This is a super-obscure corner case. However, I’ve seen it independently reported against both Chrome and Edge in the span of five years, and it makes me laugh.

If you set Chromium’s On Startup option to “Continue where you left off”, navigate to a page that tries to close itself, and then close the window, the browser will subsequently kill itself on every startup.

It’s hard to get into this state, but it’s still possible in Chrome/Edge 90.

Repro steps: Visit https://webdbg.com/test/opener/. Click the ‘Page that tries to close itself‘ link. Hit Ctrl+Shift+Delete and Clear Browsing History (to clear the back/forward stack). Close the browser with the ‘X’ icon. Now, try to launch the browser from your start menu. Giggle wildly as your browser launches and then blows itself away.

Safari/WebKit

WebKit’s code is similar to Chromium’s (unsurprising, considering their shared lineage) except it does not equate navigations triggered with noopener with those created by browser UI. Thus, in Safari, the user may click between many different same-origin pages and close() is still permitted.

If close() is blocked, Safari’s (well-hidden) JavaScript console shows:

Can't close the window since it was not opened by JavaScript

Firefox

Unlike Chromium, Firefox correctly implements the “only one Document” part of the spec. Firefox calls IsOnlyTopLevelDocumentInSHistory which calls IsEmptyOrHasEntriesForSingleTopLevelPage() which enumerates the history list to check. If there is more than one entry, whether all of the entries are for the same Document. If they are all for the same document, the close() succeeds.

Firefox offers an about:config override setting named dom.allow_scripts_to_close_windows that bypasses the default restrictions.

When Firefox blocks close(), it emits a notice to the console:

Scripts may not close windows that were not opened by script.

There’s an 18 year-old feature request for Firefox to show a confirmation dialog instead of failing silently.

In Conclusion

umm…. browsers are wicked complicated?

-Eric

Published by ericlaw

Impatient optimist. Dad. Author/speaker. Created Fiddler & SlickRun. PM @ MSFT '01-'12, and '18-, presently working on Microsoft Edge. My words are my own.

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