Break mixins.less up into smaller modules.

Move all deprecated vendor prefix mixins to less/mixins/vendor-prefixes.less
Create mixins directory and move partials to that directory.

FIXES #12994

less/mixins/alerts.less

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+// Alerts
+
+.alert-variant(@background; @border; @text-color) {
+  background-color: @background;
+  border-color: @border;
+  color: @text-color;
+
+  hr {
+    border-top-color: darken(@border, 5%);
+  }
+  .alert-link {
+    color: darken(@text-color, 10%);
+  }
+}

less/mixins/background-variant.less

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+// Contextual backgrounds
+
+.bg-variant(@color) {
+  background-color: @color;
+  a&:hover {
+    background-color: darken(@color, 10%);
+  }
+}
+

less/mixins/border-radius.less

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+// Single side border-radius
+
+.border-top-radius(@radius) {
+  border-top-right-radius: @radius;
+   border-top-left-radius: @radius;
+}
+.border-right-radius(@radius) {
+  border-bottom-right-radius: @radius;
+     border-top-right-radius: @radius;
+}
+.border-bottom-radius(@radius) {
+  border-bottom-right-radius: @radius;
+   border-bottom-left-radius: @radius;
+}
+.border-left-radius(@radius) {
+  border-bottom-left-radius: @radius;
+     border-top-left-radius: @radius;
+}

less/mixins/buttons.less

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+// Button variants
+//
+// Easily pump out default styles, as well as :hover, :focus, :active,
+// and disabled options for all buttons
+
+.button-variant(@color; @background; @border) {
+  color: @color;
+  background-color: @background;
+  border-color: @border;
+
+  &:hover,
+  &:focus,
+  &:active,
+  &.active,
+  .open .dropdown-toggle& {
+    color: @color;
+    background-color: darken(@background, 10%);
+        border-color: darken(@border, 12%);
+  }
+  &:active,
+  &.active,
+  .open .dropdown-toggle& {
+    background-image: none;
+  }
+  &.disabled,
+  &[disabled],
+  fieldset[disabled] & {
+    &,
+    &:hover,
+    &:focus,
+    &:active,
+    &.active {
+      background-color: @background;
+          border-color: @border;
+    }
+  }
+
+  .badge {
+    color: @background;
+    background-color: @color;
+  }
+}
+
+// Button sizes
+.button-size(@padding-vertical; @padding-horizontal; @font-size; @line-height; @border-radius) {
+  padding: @padding-vertical @padding-horizontal;
+  font-size: @font-size;
+  line-height: @line-height;
+  border-radius: @border-radius;
+}

less/mixins/center-block.less

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+// Center-align a block level element
+
+.center-block() {
+  display: block;
+  margin-left: auto;
+  margin-right: auto;
+}

less/mixins/clearfix.less

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+// Clearfix
+// Source: http://nicolasgallagher.com/micro-clearfix-hack/
+//
+// For modern browsers
+// 1. The space content is one way to avoid an Opera bug when the
+//    contenteditable attribute is included anywhere else in the document.
+//    Otherwise it causes space to appear at the top and bottom of elements
+//    that are clearfixed.
+// 2. The use of `table` rather than `block` is only necessary if using
+//    `:before` to contain the top-margins of child elements.
+
+.clearfix() {
+  &:before,
+  &:after {
+    content: " "; // 1
+    display: table; // 2
+  }
+  &:after {
+    clear: both;
+  }
+}
+

less/mixins/forms.less

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+// Form validation states
+//
+// Used in forms.less to generate the form validation CSS for warnings, errors,
+// and successes.
+
+.form-control-validation(@text-color: #555; @border-color: #ccc; @background-color: #f5f5f5) {
+  // Color the label and help text
+  .help-block,
+  .control-label,
+  .radio,
+  .checkbox,
+  .radio-inline,
+  .checkbox-inline  {
+    color: @text-color;
+  }
+  // Set the border and box shadow on specific inputs to match
+  .form-control {
+    border-color: @border-color;
+    .box-shadow(inset 0 1px 1px rgba(0,0,0,.075)); // Redeclare so transitions work
+    &:focus {
+      border-color: darken(@border-color, 10%);
+      @shadow: inset 0 1px 1px rgba(0,0,0,.075), 0 0 6px lighten(@border-color, 20%);
+      .box-shadow(@shadow);
+    }
+  }
+  // Set validation states also for addons
+  .input-group-addon {
+    color: @text-color;
+    border-color: @border-color;
+    background-color: @background-color;
+  }
+  // Optional feedback icon
+  .form-control-feedback {
+    color: @text-color;
+  }
+}
+
+
+
+// Forms
+// --------------------------------------------------
+
+// Form control focus state
+//
+// Generate a customized focus state and for any input with the specified color,
+// which defaults to the `@input-focus-border` variable.
+//
+// We highly encourage you to not customize the default value, but instead use
+// this to tweak colors on an as-needed basis. This aesthetic change is based on
+// WebKit's default styles, but applicable to a wider range of browsers. Its
+// usability and accessibility should be taken into account with any change.
+//
+// Example usage: change the default blue border and shadow to white for better
+// contrast against a dark gray background.
+.form-control-focus(@color: @input-border-focus) {
+  @color-rgba: rgba(red(@color), green(@color), blue(@color), .6);
+  &:focus {
+    border-color: @color;
+    outline: 0;
+    .box-shadow(~"inset 0 1px 1px rgba(0,0,0,.075), 0 0 8px @{color-rgba}");
+  }
+}
+
+// Form control sizing
+//
+// Relative text size, padding, and border-radii changes for form controls. For
+// horizontal sizing, wrap controls in the predefined grid classes. `<select>`
+// element gets special love because it's special, and that's a fact!
+.input-size(@input-height; @padding-vertical; @padding-horizontal; @font-size; @line-height; @border-radius) {
+  height: @input-height;
+  padding: @padding-vertical @padding-horizontal;
+  font-size: @font-size;
+  line-height: @line-height;
+  border-radius: @border-radius;
+
+  select& {
+    height: @input-height;
+    line-height: @input-height;
+  }
+
+  textarea&,
+  select[multiple]& {
+    height: auto;
+  }
+}

less/mixins/gradients.less

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+// Gradients
+
+// --------------------------------------------------
+
+#gradient {
+
+  // Horizontal gradient, from left to right
+  //
+  // Creates two color stops, start and end, by specifying a color and position for each color stop.
+  // Color stops are not available in IE9 and below.
+  .horizontal(@start-color: #555; @end-color: #333; @start-percent: 0%; @end-percent: 100%) {
+    background-image: -webkit-linear-gradient(left, color-stop(@start-color @start-percent), color-stop(@end-color @end-percent)); // Safari 5.1-6, Chrome 10+
+    background-image: -o-linear-gradient(left, @start-color @start-percent, @end-color @end-percent); // Opera 12
+    background-image: linear-gradient(to right, @start-color @start-percent, @end-color @end-percent); // Standard, IE10, Firefox 16+, Opera 12.10+, Safari 7+, Chrome 26+
+    background-repeat: repeat-x;
+    filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=1)",argb(@start-color),argb(@end-color))); // IE9 and down
+  }
+
+  // Vertical gradient, from top to bottom
+  //
+  // Creates two color stops, start and end, by specifying a color and position for each color stop.
+  // Color stops are not available in IE9 and below.
+  .vertical(@start-color: #555; @end-color: #333; @start-percent: 0%; @end-percent: 100%) {
+    background-image: -webkit-linear-gradient(top, @start-color @start-percent, @end-color @end-percent);  // Safari 5.1-6, Chrome 10+
+    background-image: -o-linear-gradient(top, @start-color @start-percent, @end-color @end-percent);  // Opera 12
+    background-image: linear-gradient(to bottom, @start-color @start-percent, @end-color @end-percent); // Standard, IE10, Firefox 16+, Opera 12.10+, Safari 7+, Chrome 26+
+    background-repeat: repeat-x;
+    filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=0)",argb(@start-color),argb(@end-color))); // IE9 and down
+  }
+
+  .directional(@start-color: #555; @end-color: #333; @deg: 45deg) {
+    background-repeat: repeat-x;
+    background-image: -webkit-linear-gradient(@deg, @start-color, @end-color); // Safari 5.1-6, Chrome 10+
+    background-image: -o-linear-gradient(@deg, @start-color, @end-color); // Opera 12
+    background-image: linear-gradient(@deg, @start-color, @end-color); // Standard, IE10, Firefox 16+, Opera 12.10+, Safari 7+, Chrome 26+
+  }
+  .horizontal-three-colors(@start-color: #00b3ee; @mid-color: #7a43b6; @color-stop: 50%; @end-color: #c3325f) {
+    background-image: -webkit-linear-gradient(left, @start-color, @mid-color @color-stop, @end-color);
+    background-image: -o-linear-gradient(left, @start-color, @mid-color @color-stop, @end-color);
+    background-image: linear-gradient(to right, @start-color, @mid-color @color-stop, @end-color);
+    background-repeat: no-repeat;
+    filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=1)",argb(@start-color),argb(@end-color))); // IE9 and down, gets no color-stop at all for proper fallback
+  }
+  .vertical-three-colors(@start-color: #00b3ee; @mid-color: #7a43b6; @color-stop: 50%; @end-color: #c3325f) {
+    background-image: -webkit-linear-gradient(@start-color, @mid-color @color-stop, @end-color);
+    background-image: -o-linear-gradient(@start-color, @mid-color @color-stop, @end-color);
+    background-image: linear-gradient(@start-color, @mid-color @color-stop, @end-color);
+    background-repeat: no-repeat;
+    filter: e(%("progid:DXImageTransform.Microsoft.gradient(startColorstr='%d', endColorstr='%d', GradientType=0)",argb(@start-color),argb(@end-color))); // IE9 and down, gets no color-stop at all for proper fallback
+  }
+  .radial(@inner-color: #555; @outer-color: #333) {
+    background-image: -webkit-radial-gradient(circle, @inner-color, @outer-color);
+    background-image: radial-gradient(circle, @inner-color, @outer-color);
+    background-repeat: no-repeat;
+  }
+  .striped(@color: rgba(255,255,255,.15); @angle: 45deg) {
+    background-image: -webkit-linear-gradient(@angle, @color 25%, transparent 25%, transparent 50%, @color 50%, @color 75%, transparent 75%, transparent);
+    background-image: -o-linear-gradient(@angle, @color 25%, transparent 25%, transparent 50%, @color 50%, @color 75%, transparent 75%, transparent);
+    background-image: linear-gradient(@angle, @color 25%, transparent 25%, transparent 50%, @color 50%, @color 75%, transparent 75%, transparent);
+  }
+}

less/mixins/grid-framework.less

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+// Framework grid generation
+// --------------------------------------------------
+
+// Used only by Bootstrap to generate the correct number of grid classes given
+// any value of `@grid-columns`.
+.make-grid-columns() {
+  // Common styles for all sizes of grid columns, widths 1-12
+  .col(@index) when (@index = 1) { // initial
+    @item: ~".col-xs-@{index}, .col-sm-@{index}, .col-md-@{index}, .col-lg-@{index}";
+    .col((@index + 1), @item);
+  }
+  .col(@index, @list) when (@index =< @grid-columns) { // general; "=<" isn't a typo
+    @item: ~".col-xs-@{index}, .col-sm-@{index}, .col-md-@{index}, .col-lg-@{index}";
+    .col((@index + 1), ~"@{list}, @{item}");
+  }
+  .col(@index, @list) when (@index > @grid-columns) { // terminal
+    @{list} {
+      position: relative;
+      // Prevent columns from collapsing when empty
+      min-height: 1px;
+      // Inner gutter via padding
+      padding-left:  (@grid-gutter-width / 2);
+      padding-right: (@grid-gutter-width / 2);
+    }
+  }
+  .col(1); // kickstart it
+}
+
+.float-grid-columns(@class) {
+  .col(@index) when (@index = 1) { // initial
+    @item: ~".col-@{class}-@{index}";
+    .col((@index + 1), @item);
+  }
+  .col(@index, @list) when (@index =< @grid-columns) { // general
+    @item: ~".col-@{class}-@{index}";
+    .col((@index + 1), ~"@{list}, @{item}");
+  }
+  .col(@index, @list) when (@index > @grid-columns) { // terminal
+    @{list} {
+      float: left;
+    }
+  }
+  .col(1); // kickstart it
+}
+
+.calc-grid-column(@index, @class, @type) when (@type = width) and (@index > 0) {
+  .col-@{class}-@{index} {
+    width: percentage((@index / @grid-columns));
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = push) and (@index > 0) {
+  .col-@{class}-push-@{index} {
+    left: percentage((@index / @grid-columns));
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = push) and (@index = 0) {
+  .col-@{class}-push-0 {
+    left: auto;
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = pull) and (@index > 0) {
+  .col-@{class}-pull-@{index} {
+    right: percentage((@index / @grid-columns));
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = pull) and (@index = 0) {
+  .col-@{class}-pull-0 {
+    right: auto;
+  }
+}
+.calc-grid-column(@index, @class, @type) when (@type = offset) {
+  .col-@{class}-offset-@{index} {
+    margin-left: percentage((@index / @grid-columns));
+  }
+}
+
+// Basic looping in LESS
+.loop-grid-columns(@index, @class, @type) when (@index >= 0) {
+  .calc-grid-column(@index, @class, @type);
+  // next iteration
+  .loop-grid-columns((@index - 1), @class, @type);
+}
+
+// Create grid for specific class
+.make-grid(@class) {
+  .float-grid-columns(@class);
+  .loop-grid-columns(@grid-columns, @class, width);
+  .loop-grid-columns(@grid-columns, @class, pull);
+  .loop-grid-columns(@grid-columns, @class, push);
+  .loop-grid-columns(@grid-columns, @class, offset);
+}
+