From 1d305b55114bffaece2841500c8476ba3996d2b2 Mon Sep 17 00:00:00 2001
From: David Zearing <dzearing@microsoft.com>
Date: Tue, 24 Apr 2018 19:34:37 -0700
Subject: [PATCH] Merge styles order (#4664)

* Updating how style elements are injected (to preserve the location in head in which they're inserted.)

* adding change file

* Adding nit fixes.
---
 .../merge-styles-order_2018-04-24-16-59.json  | 11 +++
 packages/merge-styles/src/Stylesheet.ts       | 72 ++++++++++---------
 2 files changed, 50 insertions(+), 33 deletions(-)
 create mode 100644 common/changes/@uifabric/merge-styles/merge-styles-order_2018-04-24-16-59.json

diff --git a/common/changes/@uifabric/merge-styles/merge-styles-order_2018-04-24-16-59.json b/common/changes/@uifabric/merge-styles/merge-styles-order_2018-04-24-16-59.json
new file mode 100644
index 0000000000000..59eca511c07b0
--- /dev/null
+++ b/common/changes/@uifabric/merge-styles/merge-styles-order_2018-04-24-16-59.json
@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "packageName": "@uifabric/merge-styles",
+      "comment": "merge-styles: `style` elements which are created on the fly now \"bunch\" together, avoiding unpredictability in specificity.",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@uifabric/merge-styles",
+  "email": "dzearing@microsoft.com"
+}
\ No newline at end of file
diff --git a/packages/merge-styles/src/Stylesheet.ts b/packages/merge-styles/src/Stylesheet.ts
index c3b1a87a52ee6..f86b551edd5c9 100644
--- a/packages/merge-styles/src/Stylesheet.ts
+++ b/packages/merge-styles/src/Stylesheet.ts
@@ -50,6 +50,7 @@ let _stylesheet: Stylesheet;
  * @public
  */
 export class Stylesheet {
+  private _lastStyleElement?: HTMLStyleElement;
   private _styleElement?: HTMLStyleElement;
   private _rules: string[] = [];
   private _config: IStyleSheetConfig;
@@ -158,28 +159,29 @@ export class Stylesheet {
   public insertRule(
     rule: string
   ): void {
-    const element = this._getElement();
-    const injectionMode = element ? this._config.injectionMode : InjectionMode.none;
-
-    switch (injectionMode) {
-      case InjectionMode.insertNode:
-        const { sheet } = element!;
-
-        try {
-          // tslint:disable-next-line:no-any
-          (sheet as any).insertRule(rule, (sheet as any).cssRules.length);
-        } catch (e) {
-          /* no-op on errors */
-        }
-        break;
-
-      case InjectionMode.appendChild:
-        _createStyleElement(rule);
-        break;
-
-      default:
-        this._rules.push(rule);
-        break;
+    const { injectionMode } = this._config;
+    const element = injectionMode !== InjectionMode.none ? this._getStyleElement() : undefined;
+
+    if (element) {
+      switch (this._config.injectionMode) {
+        case InjectionMode.insertNode:
+          const { sheet } = element!;
+
+          try {
+            (sheet as CSSStyleSheet).insertRule(rule, (sheet as CSSStyleSheet).cssRules.length);
+          } catch (e) {
+            // The browser will throw exceptions on unsupported rules (such as a moz prefix in webkit.)
+            // We need to swallow the exceptions for this scenario, otherwise we'd need to filter
+            // which could be slower and bulkier.
+          }
+          break;
+
+        case InjectionMode.appendChild:
+          element.appendChild(document.createTextNode(rule));
+          break;
+      }
+    } else {
+      this._rules.push(rule);
     }
 
     if (this._config.onInsertRule) {
@@ -212,9 +214,9 @@ export class Stylesheet {
     this._keyToClassName = {};
   }
 
-  private _getElement(): HTMLStyleElement | undefined {
+  private _getStyleElement(): HTMLStyleElement | undefined {
     if (!this._styleElement && typeof document !== 'undefined') {
-      this._styleElement = _createStyleElement();
+      this._styleElement = this._createStyleElement();
 
       // Reset the style element on the next frame.
       window.requestAnimationFrame(() => {
@@ -223,17 +225,21 @@ export class Stylesheet {
     }
     return this._styleElement;
   }
-}
 
-function _createStyleElement(content?: string): HTMLStyleElement {
-  const styleElement = document.createElement('style');
+  private _createStyleElement(): HTMLStyleElement {
+    const styleElement = document.createElement('style');
+
+    styleElement.setAttribute('data-merge-styles', 'true');
+    styleElement.type = 'text/css';
+
+    if (this._lastStyleElement && this._lastStyleElement.nextElementSibling) {
+      document.head.insertBefore(styleElement, this._lastStyleElement.nextElementSibling);
+    } else {
+      document.head.appendChild(styleElement);
+    }
+    this._lastStyleElement = styleElement;
 
-  styleElement.setAttribute('data-merge-styles', 'true');
-  styleElement.type = 'text/css';
-  if (content) {
-    styleElement.appendChild(document.createTextNode(content));
+    return styleElement;
   }
-  document.head.appendChild(styleElement);
 
-  return styleElement;
 }
\ No newline at end of file