Restore autoscroll support in reader (#4405)

- Mozilla's ActorManagerParent was only being imported when devtools
  were initialized (either by opening the Error Console or by connecting
  via remote debugging). Import it in our ActorManager so Mozilla
  actors, including AutoScroll, are always registered.
   - This may have other effects! I'm assuming most of us test with
     the error console visible, though, so we would've had Mozilla
     actors, while users running without devtools wouldn't have. We
     definitely want those actors. (Most of them, at least.)
- Don't override autoscroller styles with our panel customizations.

chrome/content/zotero/actors/ActorManager.jsm

@@ -1,5 +1,8 @@
 var EXPORTED_SYMBOLS = [];
 
+// Register Mozilla actors
+ChromeUtils.importESModule("resource://gre/modules/ActorManagerParent.sys.mjs");
+
 ChromeUtils.registerWindowActor("PageData", {
 	child: {
 		moduleURI: "chrome://zotero/content/actors/PageDataChild.jsm"

scss/components/_mainWindow.scss

@@ -1,4 +1,4 @@
-:is(panel)::part(content) {
+:is(panel):not(#autoscroller)::part(content) {
 	--panel-background: var(--material-toolbar);
 }
 

scss/win/_menupopup.scss

@@ -1,4 +1,4 @@
-menupopup, panel, #search-mode-popup {
+menupopup, panel:not(#autoscroller), #search-mode-popup {
 	--panel-border-radius: 8px;
 	--panel-background: var(--material-toolbar);
 	--menuitem-hover-background-color: var(--fill-quinary);