Follow-up to #19099 for grid fixes

- Restores two-mixin approach to generating semantic grid columns (now with 'make-col-ready' and 'make-col')
- Removes need for .col-xs-12 by restoring the mass list of all grid tier classes to set position, min-height, and padding
- Adds an initial 'width: 100%' to flexbox grid column prep (later overridden by the column sizing in 'flex' shorthand or 'width') to prevent flexbox columns from collapsing in lower viewports

docs/layout/grid.md

@@ -180,20 +180,21 @@ Mixins are used in conjunction with the grid variables to generate semantic CSS
 }
 
 // Make the element grid-ready (applying everything but the width)
-@mixin make-col($gutter: $grid-gutter-width) {
+@mixin make-col-ready($size, $columns: $grid-columns, $gutter: $grid-gutter-width) {
   position: relative;
-  @if $enable-flex {
-    flex: 1;
-  } @else {
-    float: left;
-  }
-  min-height: 1px;
-  padding-left:  ($gutter / 2);
+  min-height: 1px; // Prevent collapsing
   padding-right: ($gutter / 2);
+  padding-left:  ($gutter / 2);
+
+  // Prevent columns from becoming too narrow when at smaller grid tiers by
+  // always setting `width: 100%;`. This works because we use `flex` values
+  // later on to override this initial width.
+  @if $enable-flex {
+    width: 100%;
+  }
 }
 
-@mixin make-col-span($size, $columns: $grid-columns) {
-  // Set a width (to be used in or out of media queries)
+@mixin make-col($size, $columns: $grid-columns, $gutter: $grid-gutter-width) {
   @if $enable-flex {
     flex: 0 0 percentage($size / $columns);
     // Add a `max-width` to ensure content within each column does not blow out
@@ -201,6 +202,7 @@ Mixins are used in conjunction with the grid variables to generate semantic CSS
     // do not appear to require this.
     max-width: percentage($size / $columns);
   } @else {
+    float: left;
     width: percentage($size / $columns);
   }
 }
@@ -232,23 +234,23 @@ See it in action in <a href="http://jsbin.com/ruxona/edit">this rendered example
   @include make-row();
 }
 .content-main {
-  @include make-col();
+  @include make-col-ready();
 
   @media (max-width: 32em) {
-    @include make-col-span(6);
+    @include make-col(6);
   }
   @media (min-width: 32.1em) {
-    @include make-col-span(8);
+    @include make-col(8);
   }
 }
 .content-secondary {
-  @include make-col();
+  @include make-col-ready();
 
   @media (max-width: 32em) {
-    @include make-col-span(6);
+    @include make-col(6);
   }
   @media (min-width: 32.1em) {
-    @include make-col-span(4);
+    @include make-col(4);
   }
 }
 {% endhighlight %}

scss/mixins/_grid-framework.scss

@@ -4,22 +4,32 @@
 // any value of `$grid-columns`.
 
 @mixin make-grid-columns($columns: $grid-columns, $gutter: $grid-gutter-width, $breakpoints: $grid-breakpoints) {
+
+  // Common properties for all breakpoints
+  %grid-column {
+    position: relative;
+    // Prevent columns from collapsing when empty
+    min-height: 1px;
+    // Inner gutter via padding
+    padding-left: ($gutter / 2);
+    padding-right: ($gutter / 2);
+
+    @if $enable-flex {
+      width: 100%;
+    }
+  }
+
   $breakpoint-counter: 0;
   @each $breakpoint in map-keys($breakpoints) {
     $breakpoint-counter: ($breakpoint-counter + 1);
-    @include media-breakpoint-up($breakpoint, $breakpoints) {
-      @if $enable-flex {
-        .col-#{$breakpoint} {
-          position: relative;
-          flex-basis: 0;
-          flex-grow: 1;
-          max-width: 100%;
-          min-height: 1px;
-          padding-right: ($gutter / 2);
-          padding-left:  ($gutter / 2);
-        }
-      }
 
+    @for $i from 1 through $columns {
+      .col-#{$breakpoint}-#{$i} {
+        @extend %grid-column;
+      }
+    }
+
+    @include media-breakpoint-up($breakpoint, $breakpoints) {
       @for $i from 1 through $columns {
         .col-#{$breakpoint}-#{$i} {
           @include make-col($i, $columns, $gutter);

scss/mixins/_grid.scss

@@ -33,12 +33,21 @@
   margin-right: ($gutter / -2);
 }
 
-@mixin make-col($size, $columns: $grid-columns, $gutter: $grid-gutter-width) {
+@mixin make-col-ready($size, $columns: $grid-columns, $gutter: $grid-gutter-width) {
   position: relative;
-  min-height: 1px;
+  min-height: 1px; // Prevent collapsing
   padding-right: ($gutter / 2);
   padding-left:  ($gutter / 2);
 
+  // Prevent columns from becoming too narrow when at smaller grid tiers by
+  // always setting `width: 100%;`. This works because we use `flex` values
+  // later on to override this initial width.
+  @if $enable-flex {
+    width: 100%;
+  }
+}
+
+@mixin make-col($size, $columns: $grid-columns, $gutter: $grid-gutter-width) {
   @if $enable-flex {
     flex: 0 0 percentage($size / $columns);
     // Add a `max-width` to ensure content within each column does not blow out