Keep the mobile shell stable across tab changes

src/bro_app.cc

@@ -31,12 +31,20 @@ void BroApp::OnBeforeCommandLineProcessing(
   command_line->AppendSwitch("ignore-gpu-blocklist");
 
   // Chrome's Reading Mode ships in CEF without the profile plumbing it
-  // expects: its ReadAnythingSoftNavigationObserver dereferences a null
-  // coordinator the moment any page performs a soft navigation (History API /
-  // same-document), taking the whole browser process down. Symbolicated from
-  // a field crash on wakawaka.world, 2026-08-11 (build 85). Disabling the
-  // feature removes the observer; merge with any existing list rather than
-  // clobbering the features CEF disables on its own.
+  // expects: its page-load-metrics observer dereferences a null side-panel
+  // coordinator on ordinary page loads and soft navigations, taking the whole
+  // browser process down. Field crashes on wakawaka.world (build 85) and
+  // basement.studio (build 88).
+  //
+  // The gate is ImmersiveReadAnything, which is enabled by default. An earlier
+  // attempt disabled "ReadAnything", which is not a base::Feature at all in CEF
+  // 151 -- verified by scanning every feature struct in the shipped framework;
+  // the only "ReadAnything" string there is a SidePanelEntry::Id name. Chromium
+  // ignores unknown names in disable-features, so that switch was a no-op. If
+  // this name ever needs changing, verify it against the CEF binary first.
+  //
+  // Merge with any existing list rather than clobbering the features CEF
+  // disables on its own.
   const char kDisableFeatures[] = "disable-features";
   std::string disabled =
       command_line->HasSwitch(kDisableFeatures)
@@ -45,7 +53,7 @@ void BroApp::OnBeforeCommandLineProcessing(
   if (!disabled.empty()) {
     disabled += ",";
   }
-  disabled += "ReadAnything";
+  disabled += "ImmersiveReadAnything";
   command_line->AppendSwitchWithValue(kDisableFeatures, disabled);
 }
 

src/bro_handler.cc

@@ -147,6 +147,37 @@ void BroHandler::SetTabMobileEmulation(int browser_id, bool enabled) {
   }
 }
 
+void BroHandler::AdoptTabMobileEmulation(int browser_id) {
+  CEF_REQUIRE_UI_THREAD();
+
+  if (IsTabMobile(browser_id)) {
+    return;
+  }
+  // Recorded synchronously: the caller lays the shell and the tab's container
+  // out from this state in the same turn.
+  mobile_tab_ids_.insert(browser_id);
+
+  CefPostTask(TID_UI,
+              base::BindOnce(&BroHandler::ApplyPendingMobileEmulation, this,
+                             browser_id));
+}
+
+void BroHandler::ApplyPendingMobileEmulation(int browser_id) {
+  CEF_REQUIRE_UI_THREAD();
+
+  // The tab may have closed or been switched back to desktop while the task
+  // was queued.
+  if (!IsTabMobile(browser_id)) {
+    return;
+  }
+  CefRefPtr<CefBrowser> browser = browser_registry_.GetById(browser_id);
+  if (browser) {
+    // No reload: a freshly adopted tab has not made its first user-driven
+    // navigation yet, which is when the user agent override takes effect.
+    ApplyEmulationToBrowser(browser, /*mobile=*/true, /*reload=*/false);
+  }
+}
+
 void BroHandler::ReloadTab(int browser_id) {
   if (!CefCurrentlyOn(TID_UI)) {
     CefPostTask(TID_UI,

src/bro_handler.h

@@ -99,6 +99,13 @@ class BroHandler : public CefClient,
     return mobile_tab_ids_.count(browser_id) > 0;
   }
 
+  // Marks a just-created tab as mobile so the shell can lay it out
+  // immediately, and applies the DevTools overrides on a later UI-thread
+  // turn. Used by the adoption path, which runs inside OnAfterCreated --
+  // ExecuteDevToolsMethod on a browser still inside its creation callback
+  // dereferences null in CEF 151 (SIGSEGV on CrBrowserMain).
+  void AdoptTabMobileEmulation(int browser_id);
+
   // Reload a tab. Used to apply a pending user agent override once the
   // viewport toggle animation has finished (the UA only takes effect on the
   // next navigation).
@@ -227,6 +234,10 @@ class BroHandler : public CefClient,
                                bool mobile,
                                bool reload);
 
+  // Deferred half of AdoptTabMobileEmulation: applies the overrides unless the
+  // tab was closed or switched back to desktop in the meantime.
+  void ApplyPendingMobileEmulation(int browser_id);
+
   // True if using Alloy style (native windows)
   const bool is_alloy_style_;
 

src/bro_mac.mm

@@ -1610,8 +1610,29 @@ - (void)createApplication:(id)object {
   g_main_window.delegate = self;
   g_main_window.title = @"Bro Computer";
 
-  // Show the window
+  // Order the window while transparent so the behind-window material can
+  // attach and AppKit can commit the complete shell before any pixels become
+  // visible. Cold launches from a DMG otherwise expose the window server's
+  // black initial backing store for a frame before the glass surface resolves.
+  g_main_window.alphaValue = 0.0;
   [g_main_window makeKeyAndOrderFront:nil];
+  [g_main_window.contentView layoutSubtreeIfNeeded];
+  [g_main_window displayIfNeeded];
+  [CATransaction flush];
+
+  // Reveal without animation on the next main-queue turn, after the initial
+  // layer transaction has been submitted. Keep this independent of CEF's
+  // asynchronous browser creation so a slow or failed browser startup cannot
+  // leave the native window invisible.
+  dispatch_async(dispatch_get_main_queue(), ^{
+    if (!g_main_window) {
+      return;
+    }
+    [g_main_window.contentView layoutSubtreeIfNeeded];
+    [g_main_window displayIfNeeded];
+    [CATransaction flush];
+    g_main_window.alphaValue = 1.0;
+  });
 
   // Create the CEF browser in our window
   [self performSelectorOnMainThread:@selector(createBrowserInWindow)
@@ -2528,6 +2549,23 @@ bool OnTabCreated(int browser_id, const std::string& url, void* native_view) {
     [g_blank_tab_ids addObject:@(browser_id)];
   }
 
+  // A tab opened while a mobile-emulated tab is active inherits that mode.
+  // The shell hugs the mobile viewport for as long as such a tab is active,
+  // so a desktop-mode new tab would snap the window back to the desktop frame
+  // and re-lay the chrome on every ⌘T / + click. The outgoing tab is still
+  // the active one here: OnAfterCreated promotes this browser only after
+  // adoption returns. Applied before the tab bar work below so the toolbar's
+  // viewport toggles (updated from setActiveTab:) show the inherited mode.
+  // The DevTools overrides themselves land a turn later -- see
+  // AdoptTabMobileEmulation; issuing them from inside OnAfterCreated crashes.
+  BroHandler* creating_handler = BroHandler::GetInstance();
+  int outgoing_id =
+      creating_handler ? creating_handler->GetActiveBrowserId() : -1;
+  if (creating_handler && outgoing_id >= 0 && outgoing_id != browser_id &&
+      creating_handler->IsTabMobile(outgoing_id)) {
+    creating_handler->AdoptTabMobileEmulation(browser_id);
+  }
+
   // Add tab to tab bar
   if (g_tab_bar) {
     [g_tab_bar addTabWithBrowserId:browser_id title:kBroBlankTabTitle];
@@ -2538,8 +2576,11 @@ bool OnTabCreated(int browser_id, const std::string& url, void* native_view) {
   UpdateTabContainerVisibility(browser_id);
 
   // A newly adopted tab becomes active without going through
-  // OnActiveTabChanged; the shell follows its viewport mode (desktop).
-  UpdateWindowForViewportMode(TabIsMobile(browser_id), YES);
+  // OnActiveTabChanged; the shell follows its viewport mode (inherited from
+  // the tab it replaced, above). While split the shell stays desktop-shaped,
+  // as it does on every other tab switch.
+  UpdateWindowForViewportMode(SplitActive() ? NO : TabIsMobile(browser_id),
+                              YES);
 
   if (shouldFocusAddress) {
     // Let adoption and layout finish before asking AppKit for its field

src/bro_mac_internal.h

@@ -284,6 +284,9 @@ extern NSTextField* BroHoverCardLabel(NSFont* font, CGFloat whiteAlpha);
 // the active pill reveals it only while editing.
 @interface BroTabView : NSView
 @property (nonatomic, assign) int browserId;
+// The tab bar remains the interaction owner even though pills live inside its
+// scroll view's document view rather than as direct children.
+@property (nonatomic, weak) BroTabBar* owningTabBar;
 @property (nonatomic, strong) NSImageView* faviconView;
 @property (nonatomic, strong) BroShimmerTextView* titleLabel;
 // Each pill owns its editor for its entire lifetime. Only the active pill may
@@ -336,9 +339,10 @@ extern NSTextField* BroHoverCardLabel(NSFont* font, CGFloat whiteAlpha);
 
 #pragma mark - BroTabBar
 
-// The tab strip lives INSIDE the toolbar row: each tab is a pill (favicon +
-// host + close), the active pill hosts the editable address field, and the
-// "+" button trails the last pill.
+// The tab strip lives INSIDE the toolbar row: tab pills scroll horizontally,
+// the active pill hosts the editable address field, the "+" trails the last
+// pill until the pills overflow, and the search control stays pinned at the
+// trailing edge.
 @interface BroTabBar : NSView
 @property (nonatomic, strong) NSMutableArray<BroTabView*>* tabs;
 @property (nonatomic, strong) BroHoverButton* addTabButton;