Bug 1821985 - Use top chrome window to create select dropdown. r=Gijs

Some chrome pages can be zoomed and the select code expects coordinates
in unzoomed CSS pixels.

The easiest fix is to just use the top-level chrome window which isn't
ever zoomed to contain the select popup.

This also makes it able to use native styles on GTK / macOS / Win7.

Differential Revision: https://phabricator.services.mozilla.com/D173436

toolkit/actors/SelectParent.sys.mjs

@@ -285,8 +285,6 @@ export var SelectParentHelper = {
     this._selectRect = rect;
     this._registerListeners(menulist.menupopup);
 
-    let win = browser.ownerGlobal;
-
     // Set the maximum height to show exactly MAX_ROWS items.
     let menupopup = menulist.menupopup;
     let firstItem = menupopup.firstElementChild;
@@ -294,6 +292,7 @@ export var SelectParentHelper = {
       firstItem = firstItem.nextElementSibling;
     }
 
+    let win = menulist.ownerGlobal;
     if (firstItem) {
       let itemHeight = firstItem.getBoundingClientRect().height;
 
@@ -718,12 +717,11 @@ export var SelectParentHelper = {
 
 export class SelectParent extends JSWindowActorParent {
   get relevantBrowser() {
-    let bc = this.manager.browsingContext;
-    return bc.isContent ? bc.topFrameElement : bc.embedderElement;
+    return this.browsingContext.top.embedderElement;
   }
 
   get _document() {
-    return this.relevantBrowser.ownerDocument;
+    return this.browsingContext.topChromeWindow.document;
   }
 
   get _menulist() {
@@ -772,7 +770,7 @@ export class SelectParent extends JSWindowActorParent {
           data.selectedIndex,
           // We only want to apply the full zoom. The text zoom is already
           // applied in the font-size.
-          this.manager.browsingContext.fullZoom,
+          this.browsingContext.fullZoom,
           data.custom && lazy.CUSTOM_STYLING_ENABLED,
           data.isDarkBackground,
           data.defaultStyle,