MDL-47353 gradereport_grader: Only respect fixed headers

Even if the navbar is in a static header, it should only be respected if it
is fixed still.

This issue is a part of the MDL-46658 Task.
This issue is a part of the MDL-25544 Epic.

grade/report/grader/yui/build/moodle-gradereport_grader-gradereporttable/moodle-gradereport_grader-gradereporttable-debug.js

@@ -488,7 +488,7 @@ FloatingHeaders.prototype = {
             } else {
                 var navbar = Y.one('.navbar');
 
-                if (navbar) {
+                if (navbar && navbar.getComputedStyle('position') === 'fixed') {
                     // If the navbar exists and isn't fixed, we need to offset the page header to accommodate for it.
                     this.pageHeaderHeight = navbar.get(OFFSETHEIGHT);
                 }

grade/report/grader/yui/build/moodle-gradereport_grader-gradereporttable/moodle-gradereport_grader-gradereporttable.js

@@ -488,7 +488,7 @@ FloatingHeaders.prototype = {
             } else {
                 var navbar = Y.one('.navbar');
 
-                if (navbar) {
+                if (navbar && navbar.getComputedStyle('position') === 'fixed') {
                     // If the navbar exists and isn't fixed, we need to offset the page header to accommodate for it.
                     this.pageHeaderHeight = navbar.get(OFFSETHEIGHT);
                 }

grade/report/grader/yui/src/gradereporttable/js/floatingheaders.js

@@ -345,7 +345,7 @@ FloatingHeaders.prototype = {
             } else {
                 var navbar = Y.one('.navbar');
 
-                if (navbar) {
+                if (navbar && navbar.getComputedStyle('position') === 'fixed') {
                     // If the navbar exists and isn't fixed, we need to offset the page header to accommodate for it.
                     this.pageHeaderHeight = navbar.get(OFFSETHEIGHT);
                 }