Bug 1618678 - Make disabling layout.css.cached-scrollbar-styles.enabled not a massive perf regression. r=heycam

(Because of all the extra declarations)

Differential Revision: https://phabricator.services.mozilla.com/D65916

--HG--
extra : moz-landing-system : lando

toolkit/content/minimal-xul.css

@@ -33,60 +33,73 @@
 }
 
 /* Rules required for style caching of anonymous content scrollbar parts */
-scrollcorner:-moz-native-anonymous-no-specificity,
-resizer:-moz-native-anonymous-no-specificity,
-scrollbar:-moz-native-anonymous-no-specificity,
-scrollbarbutton:-moz-native-anonymous-no-specificity,
-slider:-moz-native-anonymous-no-specificity {
-  /* All scrollbar parts must not inherit any properties from the scrollable
-   * element (except for visibility and pointer-events), for the anonymous
-   * content style caching system to work.
-   */
-  all: initial;
-  visibility: inherit;
-  pointer-events: inherit;
+@supports -moz-bool-pref("layout.css.cached-scrollbar-styles.enabled") {
+  scrollcorner:-moz-native-anonymous-no-specificity,
+  resizer:-moz-native-anonymous-no-specificity,
+  scrollbar:-moz-native-anonymous-no-specificity,
+  scrollbarbutton:-moz-native-anonymous-no-specificity,
+  slider:-moz-native-anonymous-no-specificity {
+    /* All scrollbar parts must not inherit any properties from the scrollable
+     * element (except for visibility and pointer-events), for the anonymous
+     * content style caching system to work.
+     */
+    all: initial;
+    visibility: inherit;
+    pointer-events: inherit;
 
-  /* These properties are not included in 'all'. */
-  -moz-list-reversed: initial;
-  -moz-font-smoothing-background-color: initial;
-  -moz-min-font-size-ratio: initial;
+    /* These properties are not included in 'all'. */
+    -moz-list-reversed: initial;
+    -moz-font-smoothing-background-color: initial;
+    -moz-min-font-size-ratio: initial;
 
-  /* Using initial is not sufficient for direction, since its initial value can
-   * depend on the document's language.
-   *
-   * LTR is what we want for all scrollbar parts anyway, so that e.g. we don't
-   * reverse the rendering of a horizontal scrollbar.
-   */
-  direction: ltr;
+    /* Using initial is not sufficient for direction, since its initial value can
+     * depend on the document's language.
+     *
+     * LTR is what we want for all scrollbar parts anyway, so that e.g. we don't
+     * reverse the rendering of a horizontal scrollbar.
+     */
+    direction: ltr;
 
-  /* Similarly for font properties, whose initial values depend on the
-   * document's language.  Scrollbar parts don't have any text or rely on
-   * font metrics.
-   */
-  font: 16px sans-serif;
+    /* Similarly for font properties, whose initial values depend on the
+     * document's language.  Scrollbar parts don't have any text or rely on
+     * font metrics.
+     */
+    font: 16px sans-serif;
 
-  /* The initial value of justify-items is `legacy`, which makes it depend on
-   * the parent style.
-   *
-   * Reset it to something else.
-   */
-  justify-items: start;
+    /* The initial value of justify-items is `legacy`, which makes it depend on
+     * the parent style.
+     *
+     * Reset it to something else.
+     */
+    justify-items: start;
 
-  /* Avoid `object > *` rule in html.css from setting a useless, non-initial
-   * value of vertical-align.
-   */
-  vertical-align: initial !important;
+    /* Avoid `object > *` rule in html.css from setting a useless, non-initial
+     * value of vertical-align.
+     */
+    vertical-align: initial !important;
 
-  /* Duplicate the rules from the '*' rule above, which were clobbered by the
-   * 'all: initial' declaration.
-   *
-   * The other zero specificity rules above are on :root, and scrollbar parts
-   * cannot match :root, so no need to duplicate them.
+    /* Duplicate the rules from the '*' rule above, which were clobbered by the
+     * 'all: initial' declaration.
+     *
+     * The other zero specificity rules above are on :root, and scrollbar parts
+     * cannot match :root, so no need to duplicate them.
+     */
+    -moz-user-focus: ignore;
+    -moz-user-select: none;
+    display: -moz-box;
+    box-sizing: border-box;
+  }
+
+  /* There are other rules that set direction and cursor on scrollbar,
+   * expecting them to inherit into its children.  Explicitly inherit those,
+   * overriding the 'all: initial; direction: ltr;' declarations above.
    */
-  -moz-user-focus: ignore;
-  -moz-user-select: none;
-  display: -moz-box;
-  box-sizing: border-box;
+  scrollbarbutton:-moz-native-anonymous-no-specificity,
+  slider:-moz-native-anonymous-no-specificity,
+  thumb:-moz-native-anonymous-no-specificity {
+    direction: inherit;
+    cursor: inherit;
+  }
 }
 
 scrollbar[orient="vertical"],
@@ -104,17 +117,6 @@ thumb {
   -moz-box-pack: center;
 }
 
-/* There are other rules that set direction and cursor on scrollbar,
- * expecting them to inherit into its children.  Explicitly inherit those,
- * overriding the 'all: initial; direction: ltr;' declarations above.
- */
-scrollbarbutton:-moz-native-anonymous-no-specificity,
-slider:-moz-native-anonymous-no-specificity,
-thumb:-moz-native-anonymous-no-specificity {
-  direction: inherit;
-  cursor: inherit;
-}
-
 /********** resizer **********/
 
 resizer {