Distill the concept of the "tab top exclusion region", an area inside the tab

chrome/browser/ui/views/layout_constants.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_UI_VIEWS_LAYOUT_CONSTANTS_H_
 #define CHROME_BROWSER_UI_VIEWS_LAYOUT_CONSTANTS_H_
 
 #include "ui/gfx/geometry/insets.h"
 
 enum LayoutConstant {
@@ skipping 54 matching lines @@
   // The horizontal space between a tab's favicon and its title.
   TAB_FAVICON_TITLE_SPACING,
 
   // The maximum width we'll allow for a tab's title, when the tabstrip is wide
   // enough for tabs to take as much space as they want.
   TAB_MAXIMUM_TITLE_WIDTH,
 
   // Width available for content inside a pinned tab.
   TAB_PINNED_CONTENT_WIDTH,
 
+  // Height of the region atop the tab bounds that is normally excluded when
+  // calculating the tab hit test region or considering where to paint a
+  // custom theme background.  (Based on theme behavior it seems like at some
+  // point this was added to the area above the top of the tab images, and we
+  // now need to subtract it back off to compensate.)
+  TAB_TOP_EXCLUSION_HEIGHT,
+
   // Non-ash uses a rounded content area with no shadow in the assets.
   // Ash doesn't use a rounded content area and its top edge has an extra
   // shadow.
   TOOLBAR_CONTENT_SHADOW_HEIGHT,
   TOOLBAR_CONTENT_SHADOW_HEIGHT_ASH,
 
   // Additional horizontal padding between the elements in the toolbar.
   TOOLBAR_ELEMENT_PADDING,
 
   // Padding between the right-edge of the location bar and browser actions.
@@ skipping 28 matching lines @@
   TOOLBAR,
 
   // The spacing between a ToolbarButton's image and its border.
   TOOLBAR_BUTTON,
 };
 
 int GetLayoutConstant(LayoutConstant constant);
 gfx::Insets GetLayoutInsets(LayoutInset inset);
 
 #endif  // CHROME_BROWSER_UI_VIEWS_LAYOUT_CONSTANTS_H_