Lay out tabs to have integer widths.

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

Index: chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
diff --git a/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm b/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
index f39fffe9ece1525c06912e55f1dfdb738a2c9e9f..ef8113e85702afcc94d48a77c52c7be766d7204d 100644
--- a/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
+++ b/chrome/browser/ui/cocoa/tabs/tab_strip_controller.mm
@@ -6,6 +6,7 @@
 
 #import <QuartzCore/QuartzCore.h>
 
+#include <cmath>
 #include <limits>
 #include <string>
 
@@ -851,6 +852,7 @@ private:
   // Initialize |nonMiniTabWidth| in case there aren't any non-mini-tabs; this
   // value shouldn't actually be used.
   CGFloat nonMiniTabWidth = kMaxTabWidth;
+  CGFloat nonMiniTabWidthFraction = 0;
   const NSInteger numberOfOpenNonMiniTabs = [self numberOfOpenNonMiniTabs];
   if (numberOfOpenNonMiniTabs) {
     // Find the width of a non-mini-tab. This only applies to horizontal
@@ -862,6 +864,11 @@ private:
 
     // Clamp the width between the max and min.
     nonMiniTabWidth = MAX(MIN(nonMiniTabWidth, kMaxTabWidth), kMinTabWidth);
+
+    // Separate integral and fractional parts.
+    CGFloat integralPart = std::floor(nonMiniTabWidth);
+    nonMiniTabWidthFraction = nonMiniTabWidth - integralPart;
+    nonMiniTabWidth = integralPart;
   }
 
   BOOL visible = [[tabStripView_ window] isVisible];
@@ -870,6 +877,8 @@ private:
   bool hasPlaceholderGap = false;
   // Whether or not the last tab processed by the loop was a mini tab.
   BOOL isLastTabMini = NO;
+  CGFloat tabWidthAccumulatedFraction = 0;
+  NSInteger laidOutNonMiniTabs = 0;
   for (TabController* tab in tabArray_.get()) {
     // Ignore a tab that is going through a close animation.
     if ([closingControllers_ containsObject:tab])
@@ -920,8 +929,26 @@ private:
     // Set the width. Selected tabs are slightly wider when things get really
     // small and thus we enforce a different minimum width.
     BOOL isMini = [tab mini];
-    tabFrame.size.width = isMini ?
-        ([tab app] ? kAppTabWidth : kMiniTabWidth) : nonMiniTabWidth;
+    if (isMini) {
+      tabFrame.size.width = [tab app] ? kAppTabWidth : kMiniTabWidth;
+    } else {
+      tabFrame.size.width = nonMiniTabWidth;
+      tabWidthAccumulatedFraction += nonMiniTabWidthFraction;
+
+      if (tabWidthAccumulatedFraction >= 1.0) {

[Robert Sesek, 2011/12/13 18:46:38]

I'd maybe name and discuss the algorithm you're using.

+        ++tabFrame.size.width;
+        --tabWidthAccumulatedFraction;
+      }
+
+      // In case of rounding error, give any left over pixels to the last tab.

[Robert Sesek, 2011/12/13 18:46:38]

Why is this a "rounding error"?

[Avi (use Gerrit), 2011/12/13 19:15:01]

Assume that we have 181px to distribute across three tabs. Each tab will be
60.3333px.

First tab is 60px. Accumulate the .3333219 (or whatever the floating point
version of 1/3 is).
Second tab is 60px. Accumulate another .3333219.
By the third tab, we've accumulated .9999657. That's our missing pixel, but due
to rounding error it's not exactly 1. Since we're on our last tab, if we have
what's clearly "almost a pixel" we just treat it as it is.

[Robert Sesek, 2011/12/13 19:49:56]

Got to love floating point math.

+      if (laidOutNonMiniTabs == numberOfOpenNonMiniTabs - 1 &&
+          tabWidthAccumulatedFraction > 0.5) {
+        ++tabFrame.size.width;
+      }
+
+      ++laidOutNonMiniTabs;
+    }
+
     if ([tab selected])
       tabFrame.size.width = MAX(tabFrame.size.width, kMinSelectedTabWidth);