Fix window controls when horizontal tab bar is hidden

src/tabbar/_index.scss

@@ -39,6 +39,11 @@
 @include Option("userChrome.tabbar.on_bottom", "userChrome.tabbar.one_liner", "userChrome.hidden.tabbar") {
   @import "layout";
 }
+@include Option("userChrome.hidden.tabbar") {
+  @include NotOption("userChrome.hidden.titlebar_container") {
+    @import "layout/hidden_tabbar_window_control";
+  }
+}
 
 /*= Tab Bar - Show only current tab ==========================================*/
 @include Option("userChrome.tabbar.as_titlebar") {

src/tabbar/layout/_hidden_tabbar_window_control.scss

@@ -0,0 +1,33 @@
+// Firefox keeps a titlebar-control copy in the nav bar and normally enables it
+// only when native vertical tabs set `tabs-hidden`. Lepton hides the horizontal
+// tab bar with CSS, so opt into that copy directly.
+#nav-bar > .titlebar-buttonbox-container {
+  display: flex !important;
+}
+
+// Match the drag space at the start of the nav bar on the inner side of
+// right-aligned window controls. Firefox normally exposes this spacer only
+// when its native tab strip is hidden.
+@include WindowControl_Right {
+  :root[sizemode="normal"]
+    #toolbar-menubar[autohide][inactive]
+    ~ #nav-bar
+    > .titlebar-spacer[type="post-tabs"] {
+    display: flex !important;
+  }
+}
+
+// Lepton's existing hidden-tabbar rules expose the menubar copy, so suppress it
+// while the nav-bar copy is in use.
+#toolbar-menubar[inactive] > .titlebar-buttonbox-container {
+  display: none !important;
+}
+
+// A permanently visible menubar owns the controls, so hide the nav-bar copy
+// just as Firefox does for native vertical tabs.
+:root:not([chromehidden~="menubar"], [inFullscreen])
+  #toolbar-menubar:not([autohide], [collapsed])
+  ~ #nav-bar
+  > .titlebar-buttonbox-container {
+  display: none !important;
+}

src/tabbar/layout/_window_control_size.scss

@@ -102,9 +102,16 @@
   }
 }
 
-@include Has {
-  #navigator-toolbox:has(#toolbar-menubar[autohide="false"]) {
-    @include _remove_spacer_pre;
+@include Option("userChrome.hidden.tabbar") {
+  // The nav bar contains the actual controls, so don't reserve an additional
+  // fixed-width gap. Retain the pre-control drag space in a normal window.
+  #navigator-toolbox {
     @include _remove_spacer_post;
   }
 }
+
+// A permanently visible menu bar has no `autohide` attribute.
+#navigator-toolbox:has(#toolbar-menubar:not([autohide], [collapsed])) {
+  @include _remove_spacer_pre;
+  @include _remove_spacer_post;
+}