fixup! Bug 32308: use direct browser sizing for letterboxing.

Bug 45162: do not force letterboxing on API-initiated fullscreen.

browser/base/content/browser.css

@@ -111,7 +111,7 @@ body {
   outline: initial;
 }
 
-.letterboxing-ready .browserStack:not(.exclude-letterboxing) {
+:root:not([inDOMFullscreen]) .letterboxing-ready .browserStack:not(.exclude-letterboxing) {
   place-content: start center;
 }
 

layout/style/res/ua.css

@@ -356,8 +356,8 @@
   left: 0 !important;
   right: 0 !important;
   bottom: 0 !important;
-  width: 100%;
-  height: 100%;
+  width: 100% !important;
+  height: 100% !important;
   margin: 0 !important;
   min-width: 0 !important;
   max-width: none !important;
@@ -368,11 +368,6 @@
   transform: none !important;
 }
 
-*|*:fullscreen:not(:root, .letterboxing .browserStack:not(.exclude-letterboxing) > browser) {
-  width: 100% !important;
-  height: 100% !important;
-}
-
 xul|*:fullscreen:not(:root, [hidden="true"]) {
   /* The position: fixed; property above used to force the computed display
    * value to block. It is no longer the case now, so we manually set it here to