Add a dead zone when dragging tabs horizontally on Mac

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

Index: chrome/browser/ui/cocoa/tabs/tab_strip_drag_controller.mm
diff --git a/chrome/browser/ui/cocoa/tabs/tab_strip_drag_controller.mm b/chrome/browser/ui/cocoa/tabs/tab_strip_drag_controller.mm
index 3d168310cbb5820ab90d3ccf9d4b9e060d6e76ba..7fef8ad35fb6ea13a0b7af1092b1c05469910489 100644
--- a/chrome/browser/ui/cocoa/tabs/tab_strip_drag_controller.mm
+++ b/chrome/browser/ui/cocoa/tabs/tab_strip_drag_controller.mm
@@ -171,6 +171,12 @@ static BOOL PointIsInsideView(NSPoint screenPoint, NSView* view) {
   if (draggingWithinTabStrip_) {
     NSPoint thisPoint = [NSEvent mouseLocation];
     CGFloat offset = thisPoint.x - dragOrigin_.x;
+    // Only begin dragging the tab horizontally if it's torn out of its dead
+    // zone.
+    if (!outOfTabHorizontalDeadZone_ && fabs(offset) > kTearDistance) {

[Avi (use Gerrit), 2016/11/30 00:01:22]

Do we want kTearDistance? That's 36 pixels, and when I play on Views to see what
it does, the horizontal tear distance is a lot less.

What value does Views use here?

[erikchen, 2016/11/30 15:05:16]

Agreed that this should be significantly smaller. Given that the original bug
was about misclicks turning into horizontal drags, I'm thinking maybe 5, or even
3?

[njagsarran, 2016/11/30 17:44:49]

Looks like Views uses 10, what do you guys think of that? Still too much?

https://cs.chromium.org/chromium/src/chrome/browser/ui/views/tabs/tab_drag_co...

[njagsarran, 2016/11/30 20:26:32]

Done.

+      outOfTabHorizontalDeadZone_ = YES;
+    }
+    if (outOfTabHorizontalDeadZone_)

[Avi (use Gerrit), 2016/11/30 00:01:22]

You'd need to adjust the indentation here with a new if, and use {}.

[njagsarran, 2016/11/30 20:26:32]

Done.

     [sourceController_ insertPlaceholderForTab:[draggedTab_ tabView]
                                          frame:NSOffsetRect(sourceTabFrame_,
                                                             offset, 0)];
@@ -429,6 +435,7 @@ static BOOL PointIsInsideView(NSPoint screenPoint, NSView* view) {
       TabWindowController* dropController = sourceController_;
       [dropController moveTabViews:@[ [dropController activeTabView] ]
                     fromController:nil];
+      outOfTabHorizontalDeadZone_ = NO;

[erikchen, 2016/11/30 15:05:16]

presumably we always want to reset this when the drag ends.

[njagsarran, 2016/11/30 20:26:32]

Done.

     }
   } else if (targetController_) {
     // Move between windows. If |targetController_| is nil, we're not dropping