Follow El Capitan Dock settings for double-click in window title bar.

ui/base/cocoa/appkit_utils.mm

Index: ui/base/cocoa/appkit_utils.mm
diff --git a/ui/base/cocoa/appkit_utils.mm b/ui/base/cocoa/appkit_utils.mm
index 6aa90ae6789d1dcb58f9b87ee753d1576db4f1f0..66ae924e12963426672f2886667c40dc45e5bc28 100644
--- a/ui/base/cocoa/appkit_utils.mm
+++ b/ui/base/cocoa/appkit_utils.mm
@@ -29,6 +29,22 @@ bool ShouldWindowsMiniaturizeOnDoubleClick() {
          [NSWindow performSelector:@selector(_shouldMiniaturizeOnDoubleClick)];
 }
 
+// Whether windows should maximize on a double-click on the title bar.
+bool ShouldWindowsMaximizeOnDoubleClick() {
+  // Pre-Yosmite the answer was "no."
+  if (!base::mac::IsOSYosemiteOrLater()) {

[tapted, 2016/01/17 23:53:18]

nit: IsOSMavericksOrEarlier() ?

[shrike, 2016/01/19 19:34:23]

I guess I like !IsOSYosemiteOrLater() because the code is saying for everything
before Yosemite, return false. Saying 

  if (base::mac::IsOSMavericksOrEarlier()) {
    return false;
  } else if (base::mac::IsOSYosemite()) {
    return true;
  }

means that everything after Yosemite, and in between Mavericks and Yosemite,
falls through. I myself have a little trouble remembering the OSes in between
MountainLion and Yosemite, so to me my version is more clear.

[tapted, 2016/01/19 22:16:34]

Makes sense, and I agree with your point. (and I'm truly not fussed, but an
option to get the same "everything else" logic could be to do the El Capitan
check first, similar to ShouldWindowsMiniaturize, then ShouldWindowsMaximize
could finish up as simply `return base::mac::IsOSYosemite();`)

[shrike, 2016/01/20 17:23:54]

Yeah, that makes a lot of sense, and has nice symmetry.

I thought about your comment on making a single preferences read. Originally I
half believed that these ShouldMin/Max functions were needed elsewhere but given
that they both are only used once in AppkitUtils I took a stab at combining
them. The logic is a little hairier than before but it's basically the same: at
the start I do the special El Capitan work, and then I check ShouldMinimize and
then ShouldMaximize, just as WindowTitlebarReceivedDoubleClick() did before.

PTAL

+    return false;
+  } else if (base::mac::IsOSYosemite()) {
+    return true;
+  }
+
+  // In El Capitan there's a Dock setting that governs the action.

[tapted, 2016/01/17 23:53:18]

nit: Maybe something like "El Capitan introduced a checkbox to disable the
double-click action, as well as switch between Minimize and Zoom."  (otherwise
it's confusing why we don't select for Zoom here as well -- e.g. maybe we
shouldn't use ShouldWindows*Miniaturize*OnDoubleClick() on El Capitan at all,
but what you have seems neat enough. It looks like they haven't killed the
_shouldMiniaturizeOnDoubleClick method yet.)

[shrike, 2016/01/19 19:34:23]

Good point. I added your comment language, and also changed
ShouldWindowsMiniaturizeOnDoubleClick() to read the Dock preference under El
Capitan. Originally I decided against doing that because the undocumented method
seems to still work, but explicitly checking the preference on El Capitan and
above is more clear, and future proofs us against Apple removing the
undocumented API.

+  NSString* doubleClickAction = [[NSUserDefaults standardUserDefaults]
+                                 objectForKey:@"AppleActionOnDoubleClick"];
+
+  return [doubleClickAction isEqualToString:@"Maximize"];
+}
+
 }  // namespace
 
 namespace ui {
@@ -56,7 +72,7 @@ void DrawNinePartImage(NSRect frame,
 void WindowTitlebarReceivedDoubleClick(NSWindow* window, id sender) {
   if (ShouldWindowsMiniaturizeOnDoubleClick()) {
     [window performMiniaturize:sender];
-  } else if (base::mac::IsOSYosemiteOrLater()) {
+  } else if (ShouldWindowsMaximizeOnDoubleClick()) {
     [window performZoom:sender];
   }
 }