fix #10257

dist/css/bootstrap-theme.css

@@ -208,6 +208,7 @@
   background-repeat: repeat-x;
   border-radius: 4px;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ffffffff', endColorstr='#fff8f8f8', GradientType=0);
+  filter: progid:DXImageTransform.Microsoft.gradient(enabled=false);
   -webkit-box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.15), 0 1px 5px rgba(0, 0, 0, 0.075);
           box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.15), 0 1px 5px rgba(0, 0, 0, 0.075);
 }
@@ -235,6 +236,7 @@
   background-image: linear-gradient(to bottom, #3c3c3c 0%, #222222 100%);
   background-repeat: repeat-x;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ff3c3c3c', endColorstr='#ff222222', GradientType=0);
+  filter: progid:DXImageTransform.Microsoft.gradient(enabled=false);
 }
 
 .navbar-inverse .navbar-nav > .active > a {

less/theme.less

@@ -104,6 +104,7 @@
 // Default navbar
 .navbar-default {
   #gradient > .vertical(@start-color: lighten(@navbar-default-bg, 10%); @end-color: @navbar-default-bg);
+  .reset-filter(); // Remove gradient in IE<10 to fix bug where dropdowns don't get triggered
   border-radius: @navbar-border-radius;
   @shadow: inset 0 1px 0 rgba(255,255,255,.15), 0 1px 5px rgba(0,0,0,.075);
   .box-shadow(@shadow);
@@ -121,6 +122,7 @@
 // Inverted navbar
 .navbar-inverse {
   #gradient > .vertical(@start-color: lighten(@navbar-inverse-bg, 10%); @end-color: @navbar-inverse-bg);
+  .reset-filter(); // Remove gradient in IE<10 to fix bug where dropdowns don't get triggered
 
   .navbar-nav > .active > a {
     #gradient > .vertical(@start-color: @navbar-inverse-bg; @end-color: lighten(@navbar-inverse-bg, 2.5%));