Add more variables

sass/elements/button.sass

@@ -269,15 +269,16 @@ $button-static-border-color: $grey-lighter !default
           z-index: 4
       &.is-expanded
         flex-grow: 1
+        flex-shrink: 1
   &.is-centered
     justify-content: center
-    &:not(.has-addons) 
+    &:not(.has-addons)
       .button:not(.is-fullwidth)
         margin-left: 0.25rem
         margin-right: 0.25rem
   &.is-right
     justify-content: flex-end
-    &:not(.has-addons) 
+    &:not(.has-addons)
       .button:not(.is-fullwidth)
         margin-left: 0.25rem
         margin-right: 0.25rem

sass/elements/container.sass

@@ -1,21 +1,23 @@
+$container-offset: (2 * $gap) !default
+
 .container
   flex-grow: 1
   margin: 0 auto
   position: relative
   width: auto
   +desktop
-    max-width: $desktop - (2 * $gap)
+    max-width: $desktop - $container-offset
     &.is-fluid
       margin-left: $gap
       margin-right: $gap
       max-width: none
   +until-widescreen
     &.is-widescreen
-      max-width: $widescreen - (2 * $gap)
+      max-width: $widescreen - $container-offset
   +until-fullhd
     &.is-fullhd
-      max-width: $fullhd - (2 * $gap)
+      max-width: $fullhd - $container-offset
   +widescreen
-    max-width: $widescreen - (2 * $gap)
+    max-width: $widescreen - $container-offset
   +fullhd
-    max-width: $fullhd - (2 * $gap)
+    max-width: $fullhd - $container-offset

sass/elements/form.sass

@@ -471,6 +471,7 @@ $help-size: $size-small !default
               z-index: 4
       &.is-expanded
         flex-grow: 1
+        flex-shrink: 1
     &.has-addons-centered
       justify-content: center
     &.has-addons-right