--- a/browser/components/customizableui/PanelMultiView.jsm
+++ b/browser/components/customizableui/PanelMultiView.jsm
@@ -162,19 +162,16 @@ this.PanelMultiView = class {
   }
   set _mainViewId(val) {
     this.node.setAttribute("mainViewId", val);
     return val;
   }
   get _mainView() {
     return this._mainViewId ? this.document.getElementById(this._mainViewId) : null;
   }
-  get showingSubViewAsMainView() {
-    return this.node.getAttribute("mainViewIsSubView") == "true";
-  }
 
   get _transitioning() {
     return this.__transitioning;
   }
   set _transitioning(val) {
     this.__transitioning = val;
     if (val) {
       this.node.setAttribute("transitioning", "true");
@@ -455,50 +452,52 @@ this.PanelMultiView = class {
       let detail = {
         blockers: new Set(),
         addBlocker(aPromise) {
           this.blockers.add(aPromise);
         },
       };
 
       // Make sure that new panels always have a title set.
+      let cancel = false;
       if (this.panelViews && aAnchor) {
         if (aAnchor && !viewNode.hasAttribute("title"))
           viewNode.setAttribute("title", aAnchor.getAttribute("label"));
         viewNode.classList.add("PanelUI-subView");
         let custWidget = CustomizableWidgets.find(widget => widget.viewId == viewNode.id);
         if (custWidget) {
           if (custWidget.onInit)
             custWidget.onInit(aAnchor);
-          custWidget.onViewShowing({ target: aAnchor, detail });
+          custWidget.onViewShowing({ target: viewNode, preventDefault: () => cancel = true, detail });
         }
       }
-      viewNode.setAttribute("current", true);
       if (this.panelViews && this._mainViewWidth)
         viewNode.style.maxWidth = viewNode.style.minWidth = this._mainViewWidth + "px";
 
       let evt = new window.CustomEvent("ViewShowing", { bubbles: true, cancelable: true, detail });
       viewNode.dispatchEvent(evt);
 
-      let cancel = evt.defaultPrevented;
+      if (!cancel)
+        cancel = evt.defaultPrevented;
       if (detail.blockers.size) {
         try {
           let results = await Promise.all(detail.blockers);
           cancel = cancel || results.some(val => val === false);
         } catch (e) {
           Cu.reportError(e);
           cancel = true;
         }
       }
 
       if (cancel) {
         return;
       }
 
       this._currentSubView = viewNode;
+      viewNode.setAttribute("current", true);
       if (this.panelViews) {
         this.node.setAttribute("viewtype", "subview");
         if (!playTransition)
           this.descriptionHeightWorkaround(viewNode);
       }
 
       // Now we have to transition the panel. There are a few parts to this:
       //
@@ -620,29 +619,31 @@ this.PanelMultiView = class {
 
                 if (!reverse)
                   viewNode.style.removeProperty("margin-inline-start");
                 if (aAnchor)
                   aAnchor.removeAttribute("open");
 
                 this._viewContainer.removeAttribute("transition-reverse");
 
-                evt = new window.CustomEvent("ViewShown", { bubbles: true, cancelable: false });
-                viewNode.dispatchEvent(evt);
+                viewNode.dispatchEvent(new window.CustomEvent("ViewShown",
+                  { bubbles: true, cancelable: false }));
               }, { once: true });
             });
           }, { once: true });
         });
       } else if (!this.panelViews) {
         this._transitionHeight(() => {
           viewNode.setAttribute("current", true);
           this.node.setAttribute("viewtype", "subview");
           // Now that the subview is visible, we can check the height of the
           // description elements it contains.
           this.descriptionHeightWorkaround(viewNode);
+          viewNode.dispatchEvent(new window.CustomEvent("ViewShown",
+            { bubbles: true, cancelable: false }));
         });
         this._shiftMainView(aAnchor);
       }
     })().catch(e => Cu.reportError(e));
   }
 
   /**
    * Calculate the correct bounds of a panelview node offscreen to minimize the

--- a/browser/components/customizableui/content/panelUI.js
+++ b/browser/components/customizableui/content/panelUI.js
@@ -510,22 +510,24 @@ const PanelUI = {
       tempPanel.classList.toggle("cui-widget-panelWithFooter",
                                  viewNode.querySelector(".panel-subview-footer"));
 
       let multiView = document.createElement(gPhotonStructure ? "photonpanelmultiview" : "panelmultiview");
       multiView.setAttribute("id", "customizationui-widget-multiview");
       multiView.setAttribute("nosubviews", "true");
       multiView.setAttribute("viewCacheId", "appMenu-viewCache");
       if (gPhotonStructure) {
+        tempPanel.setAttribute("photon", true);
         multiView.setAttribute("mainViewId", viewNode.id);
         multiView.appendChild(viewNode);
       }
       tempPanel.appendChild(multiView);
-      multiView.setAttribute("mainViewIsSubView", "true");
-      multiView.setMainView(viewNode);
+      if (!gPhotonStructure) {
+        multiView.setMainView(viewNode);
+      }
       viewNode.classList.add("cui-widget-panelview");
 
       let viewShown = false;
       let panelRemover = () => {
         viewNode.classList.remove("cui-widget-panelview");
         if (viewShown) {
           CustomizableUI.removePanelCloseListeners(tempPanel);
           tempPanel.removeEventListener("popuphidden", panelRemover);

--- a/browser/components/customizableui/test/browser_947914_button_history.js
+++ b/browser/components/customizableui/test/browser_947914_button_history.js
@@ -9,17 +9,19 @@ add_task(async function() {
   info("Check history button existence and functionality");
 
   await PanelUI.show();
   info("Menu panel was opened");
 
   let historyButton = document.getElementById("history-panelmenu");
   ok(historyButton, "History button appears in Panel Menu");
 
+  let historyPanel = document.getElementById("PanelUI-history");
+  let promise = BrowserTestUtils.waitForEvent(historyPanel, "ViewShown");
   historyButton.click();
-  let historyPanel = document.getElementById("PanelUI-history");
+  await promise;
   ok(historyPanel.getAttribute("current"), "History Panel is in view");
 
   let panelHiddenPromise = promisePanelHidden(window);
   PanelUI.hide();
   await panelHiddenPromise
   info("Menu panel was closed");
 });

--- a/browser/components/extensions/ext-browserAction.js
+++ b/browser/components/extensions/ext-browserAction.js
@@ -125,16 +125,17 @@ this.browserAction = class extends Exten
       label: this.defaults.title || this.extension.name,
       tooltiptext: this.defaults.title || "",
       defaultArea: this.defaults.area,
 
       onBeforeCreated: document => {
         let view = document.createElementNS(XUL_NS, "panelview");
         view.id = this.viewId;
         view.setAttribute("flex", "1");
+        view.setAttribute("extension", true);
 
         document.getElementById("PanelUI-multiView").appendChild(view);
         document.addEventListener("popupshowing", this);
       },
 
       onDestroyed: document => {
         document.removeEventListener("popupshowing", this);
 
@@ -166,16 +167,20 @@ this.browserAction = class extends Exten
         let popupURL = this.getProperty(tab, "popup");
         this.tabManager.addActiveTabPermission(tab);
 
         // Popups are shown only if a popup URL is defined; otherwise
         // a "click" event is dispatched. This is done for compatibility with the
         // Google Chrome onClicked extension API.
         if (popupURL) {
           try {
+            // FIXME: The line below needs to change eventually, but for now:
+            // ensure the view is _always_ visible _before_ `popup.attach()` is
+            // called. PanelMultiView.jsm dictates different behavior.
+            event.target.setAttribute("current", true);
             let popup = this.getPopup(document.defaultView, popupURL);
             event.detail.addBlocker(popup.attach(event.target));
           } catch (e) {
             Cu.reportError(e);
             event.preventDefault();
           }
         } else {
           // This isn't not a hack, but it seems to provide the correct behavior

--- a/browser/components/extensions/test/browser/browser_ext_popup_corners.js
+++ b/browser/components/extensions/test/browser/browser_ext_popup_corners.js
@@ -1,13 +1,15 @@
 /* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
 /* vim: set sts=2 sw=2 et tw=80: */
 "use strict";
 
 add_task(async function testPopupBorderRadius() {
+  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
+
   let extension = ExtensionTestUtils.loadExtension({
     background() {
       browser.tabs.query({active: true, currentWindow: true}, tabs => {
         browser.pageAction.show(tabs[0].id);
       });
     },
 
     manifest: {

--- a/browser/themes/shared/customizableui/panelUI.inc.css
+++ b/browser/themes/shared/customizableui/panelUI.inc.css
@@ -256,17 +256,18 @@ photonpanelmultiview .panel-subview-head
 }
 
 #appMenu-popup > arrowscrollbox > scrollbox,
 #PanelUI-popup > arrowscrollbox > scrollbox {
   overflow: visible;
 }
 
 #appMenu-popup > .panel-arrowcontainer > .panel-arrowcontent,
-#PanelUI-popup > .panel-arrowcontainer > .panel-arrowcontent {
+#PanelUI-popup > .panel-arrowcontainer > .panel-arrowcontent,
+panel[photon] > .panel-arrowcontainer > .panel-arrowcontent {
   overflow: hidden;
 }
 
 #PanelUI-popup > .panel-arrowcontainer > .panel-arrowcontent,
 .cui-widget-panel > .panel-arrowcontainer > .panel-arrowcontent > .popup-internal-box {
   padding: 0;
 }
 
@@ -333,27 +334,29 @@ panelview:not([mainview]) .toolbarbutton
 }
 
 .cui-widget-panel > .panel-arrowcontainer > .panel-arrowcontent {
   padding: 4px 0;
 }
 
 /* START photonpanelview adjustments */
 
-#appMenu-popup > .panel-arrowcontainer > .panel-arrowcontent {
+#appMenu-popup > .panel-arrowcontainer > .panel-arrowcontent,
+panel[photon] > .panel-arrowcontainer > .panel-arrowcontent {
   padding: 0;
   border-radius: 0;
 }
 
 photonpanelmultiview panelview {
   background: var(--arrowpanel-background);
   padding: 6px 0;
 }
 
-#appMenu-popup panelview {
+#appMenu-popup panelview,
+#customizationui-widget-multiview panelview:not([extension]) {
   min-width: @menuPanelWidth@;
 }
 
 photonpanelmultiview panelview[title] {
   padding-top: 0;
 }
 
 /* END photonpanelview adjustments */
@@ -2071,9 +2074,19 @@ photonpanelmultiview .PanelUI-subView .p
   display: none;
 }
 
 photonpanelmultiview .PanelUI-subView toolbarseparator {
   margin-inline-start: 0;
   margin-inline-end: 0;
 }
 
+photonpanelmultiview#customizationui-widget-multiview > .panel-viewcontainer {
+  overflow: hidden;
+}
+
+/* This is explicitly overriding the overflow properties set above. */
+photonpanelmultiview .cui-widget-panelview {
+  overflow-x: visible;
+  overflow-y: visible;
+}
+
 /* END photon adjustments */