Fix tabs are not rendered properly when restore tabs after crash on iPad

ios/chrome/browser/ui/tabs/tab_strip_controller.mm

Index: ios/chrome/browser/ui/tabs/tab_strip_controller.mm
diff --git a/ios/chrome/browser/ui/tabs/tab_strip_controller.mm b/ios/chrome/browser/ui/tabs/tab_strip_controller.mm
index 715b40e8287fe44be710e795880c888358ca1083..623981a7afaf6b570d18e16c8c1cf84d51a6941d 100644
--- a/ios/chrome/browser/ui/tabs/tab_strip_controller.mm
+++ b/ios/chrome/browser/ui/tabs/tab_strip_controller.mm
@@ -1263,6 +1263,30 @@ - (void)updateContentOffsetForTabIndex:(NSUInteger)tabIndex
     // long. The amount of scroll is calculated as a desired length that it is
     // just large enough to contain all the tabs to the left of |tabIndex|, with
     // the standard overlap.
+    if (tabIndex == [_tabArray count] - 1) {

[rohitrao (ping after 24h), 2017/01/19 01:44:22]

If crash restore adds 100 tabs to the tabstrip, will we shift every tab view 100
times?  If you try with a smaller number of tabs, say 20, is there a noticeable
delay in restoring?

[liaoyuke, 2017/01/20 00:55:51]

Yes, the complexity is n^2, every tab view will be shifted 100 times.

I didn't see any noticeable delay for 20 tabs. While for 100 tabs, there is a
noticeable delay both with or without the change, maybe a little bit longer with
the change.

+      const CGFloat tabStripAvailableSpace =
+          _tabStripView.frame.size.width - _tabStripView.contentInset.right;
+      CGPoint oldOffset = [_tabStripView contentOffset];
+      if (_tabStripView.contentSize.width > tabStripAvailableSpace) {
+        CGFloat scrollToPoint =
+            _tabStripView.contentSize.width - tabStripAvailableSpace;
+        [_tabStripView setContentOffset:CGPointMake(scrollToPoint, 0)];
+      }
+
+      // To handle content offset change without making views appear to jump,
+      // shift all of the subviews by an amount equal to the size change.  This
+      // effectively places the subviews back where they were before the change,
+      // in terms of screen coordinates.
+      CGFloat dx = [_tabStripView contentOffset].x - oldOffset.x;
+      for (UIView* view in [_tabStripView subviews]) {

[rohitrao (ping after 24h), 2017/01/19 01:44:22]

Is there any way to share this code with the other callsite at line 1200 above?

[liaoyuke, 2017/01/20 00:55:51]

Done.

+        CGRect frame = [view frame];
+        frame.origin.x += dx;
+        [view setFrame:frame];
+        _targetFrames.AddFrame(view, frame);
+      }
+      return;

[rohitrao (ping after 24h), 2017/01/19 01:44:22]

This function now has two early returns, which makes me think there's probably a
better way to organize the code.  Should we split this up into three separate
functions, and call one of the three based on which case we are in?

[liaoyuke, 2017/01/20 00:55:51]

From my point of view, I think it would be non-trivial to come up with good
names for those three functions, I tried to think about some names, but I found
them too confusing. Please let me know if you have some good names in mind.

I agree that we should have a better way to organize the code, so I tentatively
moved the new code into a separate function while left the old code unchanged.
Once the old code is deleted, we can move it back. Please let me know how you
think about this.

+    }
+
     NSUInteger numNonClosingTabsToLeft = 0;
     NSUInteger i = 0;
     for (TabView* tab in _tabArray.get()) {