Bug 1186244 - Remove border artifacts between navbar and tabbar in lw-themes on Windows 10. r=dao

browser/themes/shared/tabs.inc.css

@@ -12,6 +12,7 @@
   --tab-separator-image: url(chrome://browser/skin/tabbrowser/tab-separator.png);
   --tab-separator-size: 3px 100%;
   --tab-separator-opacity: 1;
+  --tab-stroke-background-size: auto 100%;
 }
 
 %define tabCurveWidth 30px
@@ -376,7 +377,7 @@
                     @fgTabTexture@,
                     none;
   background-repeat: repeat-x;
-  background-size: auto 100%;
+  background-size: var(--tab-stroke-background-size), auto 100%;
   /* The padding-top combined with background-clip: content-box (the bottom-most) ensure the
      background-color doesn't extend above the top border. */
   padding-top: 2px;
@@ -390,7 +391,7 @@
                     @fgTabTextureLWT@;/*,
                     lwtHeader;*/
   /* Don't stretch the LWT header images */
-  background-size: auto 100%, auto 100%, auto auto;
+  background-size: var(--tab-stroke-background-size), auto 100%, auto auto;
 }
 
 /* These LWT styles are normally overridden by browser-lightweightTheme.css */

browser/themes/windows/browser.css

@@ -317,7 +317,6 @@
 #nav-bar {
   background-clip: padding-box;
   background-image: linear-gradient(@toolbarHighlight@, transparent);
-  box-shadow: 0 1px 0 @toolbarHighlight@ inset;
 }
 
 @media (-moz-os-version: windows-xp),
@@ -326,6 +325,7 @@
        (-moz-os-version: windows-win8) {
   #nav-bar {
     border-top: 1px solid @toolbarShadowColor@ !important;
+    box-shadow: 0 1px 0 @toolbarHighlight@ inset;
   }
   @media not all and (-moz-windows-compositor) {
     #TabsToolbar[collapsed="true"] + #nav-bar {
@@ -2058,10 +2058,8 @@ richlistitem[type~="action"][actiontype="switchtab"] > .ac-url-box > .ac-action-
           content: none;
         }
 
-        .tab-background-middle[visuallyselected=true] {
-          /* Setting background-size to "0 0" for the first
-             background-image to remove the stroke. */
-          background-size: 0 0, auto 100%, auto 100%;
+        #TabsToolbar {
+          --tab-stroke-background-size: 0 0;
         }
 
         :root {