Start a drag if the tab is dragged off the sides of the window....

chrome/browser/cocoa/tab_strip_controller.h

Index: chrome/browser/cocoa/tab_strip_controller.h
===================================================================
--- chrome/browser/cocoa/tab_strip_controller.h	(revision 25753)
+++ chrome/browser/cocoa/tab_strip_controller.h	(working copy)
@@ -120,6 +120,12 @@
                           frame:(NSRect)frame
                   yStretchiness:(CGFloat)yStretchiness;
 
+// Returns whether or not |tab| can still be fully seen in the tab strip or if
+// its current position would cause it be obscured by things such as the edge
+// of the window or the window decorations. Returns YES only if the entire tab
+// is visible.
+- (BOOL)isTabFullyVisible:(TabView*)tab;
+
 // Show or hide the new tab button. The button is hidden immediately, but
 // waits until the next call to |-layoutTabs| to show it again.
 - (void)showNewTabButton:(BOOL)show;