When Apple released the Safari 4 public beta at the end of February, I discovered an annoying bug: the little Javascript bookmarklet that I use to maximize the browser window didn’t work anymore. I find the behavior of Safari’s green zoom button extremely annoying. It doesn’t expand the browser window to fill the screen, but instead only resizes it to fit the content of the current page. This is typically some weird height and width that causes every subsequent page you view scroll vertically and horizontally.

Of course, Apple’s Human Interface Guidelines for Resizing and Zooming Windows explain that the zoom button isn’t actually supposed to expand the window to fill the screen, but toggle the window between a standard application-defined size and a user-defined size. Regardless of what the human interface guidelines say, though, I still don’t like what it does, and I’m apparently not the only person. There are hundreds of pages asking how to fix it.

In case you’re wondering, the bookmarklet is:

I simply put the bookmarklet first in my bookmark bar, which lets me maximize Safari by hitting ⌘1. I didn’t write this little snippet of Javascript, but I can’t remember where I found it and there are versions of it all over the Web.

So, back to the real story. Upon further investigation, I determined that the bookmarklet only works when the Safari window contains a single tab. If the window has more than one tab, the bookmarklet doesn’t work. I wasn’t the only one who noticed this bug, as I found the post Safari 4 breaks bookmarklets? on This Is the Green Room.

I figured the problem would be fixed when the final version of Safari 4 was released, but when that happened this past week, the problem remained. Googling a bit, I found WebKit bug 24218, which notes that the Javascript window.moveTo and resizeTo methods do not work in Safari windows with more than one tab.

Since the bug was marked unconfirmed and didn’t have any activity on it for months, I decided to download the WebKit source and try to track down the cause of the problem. I checked out the WebKit code and built it according to the instructions. After digging a bit and setting some breakpoints, I figured out that the following method in WebCore/page/DOMWindow.cpp implements window.moveTo in the Javascript engine:

This method basically does some adjustments with the screen and window rectangles, and then finally invokes Chrome::setWindowRect to perform the actual move of the window. Stepping through the method when clicking the bookmarklet with one and two tabs showed that all the rectangle calculations were resulting in the same values regardless of the number of tabs. The invocation of setWindowRect simply wasn’t moving the window when there was more than one tab.

After a bit of indirection through some other classes, Chrome::setWindowRect calls WebChromeClient::setWindowRect in mac/WebCoreSupport/WebChromeClient.mm:

This method basically translates the rectangle coordinates between the scale used by the window and the WebView itself, and then sends setFrame:windowRect: to WebUIDelegate. This delegate is the bridge between WebKit’s Javascript engine and the actual Cocoa browser window (I think), and WebKit is passing it the correct coordinates to move the window. WebUIDelegate, though, simply doesn’t move the window as it should when there is more than one tab, which leads me to believe the bug is in Safari itself, not WebKit.

I updated the WebKit bug and filed a new bug for Safari on Radar. I thought for a moment that this behavior might actually be intentional, to prevent pages from resizing the browser window via Javascript when there are other open tabs that would be affected. But since the bookmarklet works in Safari 3, I tend to doubt this new behavior is by design as it would break a lot of existing Javascript. Scripts embedded in an actual page are similarly unable to move or resize the window when there’s more than one tab open, so the problem doesn’t have anything to do with the Javascript being in a bookmarklet.

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