diff --git a/Userland/Applications/HexEditor/HexEditorWindow.gml b/Userland/Applications/HexEditor/HexEditorWindow.gml
index e673aff0998..0b7a4e89ee8 100644
--- a/Userland/Applications/HexEditor/HexEditorWindow.gml
+++ b/Userland/Applications/HexEditor/HexEditorWindow.gml
@@ -14,6 +14,8 @@
     }
 
     @GUI::HorizontalSplitter {
+        opportunistic_resizee: "First"
+
         @HexEditor::HexEditor {
             name: "editor"
         }
diff --git a/Userland/Applications/Mail/MailWindow.gml b/Userland/Applications/Mail/MailWindow.gml
index 602bf7d609f..5f6cadd85e8 100644
--- a/Userland/Applications/Mail/MailWindow.gml
+++ b/Userland/Applications/Mail/MailWindow.gml
@@ -7,7 +7,7 @@
     @GUI::HorizontalSplitter {
         @GUI::TreeView {
             name: "mailbox_list"
-            fixed_width: 250
+            preferred_width: 250
         }
 
         @GUI::VerticalSplitter {
diff --git a/Userland/Applications/PDFViewer/PDFViewerWidget.cpp b/Userland/Applications/PDFViewer/PDFViewerWidget.cpp
index 0f388d922d3..73600f38674 100644
--- a/Userland/Applications/PDFViewer/PDFViewerWidget.cpp
+++ b/Userland/Applications/PDFViewer/PDFViewerWidget.cpp
@@ -28,9 +28,11 @@ PDFViewerWidget::PDFViewerWidget()
     auto& toolbar = toolbar_container.add<GUI::Toolbar>();
 
     auto& splitter = add<GUI::HorizontalSplitter>();
+    splitter.layout()->set_spacing(4);
 
     m_sidebar = splitter.add<SidebarWidget>();
-    m_sidebar->set_fixed_width(0);
+    m_sidebar->set_preferred_width(200);
+    m_sidebar->set_visible(false);
 
     m_viewer = splitter.add<PDFViewer>();
     m_viewer->on_page_change = [&](auto new_page) {
@@ -73,7 +75,7 @@ void PDFViewerWidget::initialize_toolbar(GUI::Toolbar& toolbar)
     auto open_outline_action = GUI::Action::create(
         "Toggle &Sidebar", { Mod_Ctrl, Key_S }, Gfx::Bitmap::try_load_from_file("/res/icons/16x16/sidebar.png"sv).release_value_but_fixme_should_propagate_errors(), [&](auto&) {
             m_sidebar_open = !m_sidebar_open;
-            m_sidebar->set_fixed_width(m_sidebar_open ? 200 : 0);
+            m_sidebar->set_visible(m_sidebar_open ? true : false);
         },
         nullptr);
     open_outline_action->set_enabled(false);
@@ -214,11 +216,11 @@ void PDFViewerWidget::open_file(Core::File& file)
     if (document->outline()) {
         auto outline = document->outline();
         m_sidebar->set_outline(outline.release_nonnull());
-        m_sidebar->set_fixed_width(200);
+        m_sidebar->set_visible(true);
         m_sidebar_open = true;
     } else {
         m_sidebar->set_outline({});
-        m_sidebar->set_fixed_width(0);
+        m_sidebar->set_visible(false);
         m_sidebar_open = false;
     }
 }
diff --git a/Userland/Applications/SoundPlayer/SoundPlayerWidgetAdvancedView.cpp b/Userland/Applications/SoundPlayer/SoundPlayerWidgetAdvancedView.cpp
index b05d97119ec..1a0d9cdaeb8 100644
--- a/Userland/Applications/SoundPlayer/SoundPlayerWidgetAdvancedView.cpp
+++ b/Userland/Applications/SoundPlayer/SoundPlayerWidgetAdvancedView.cpp
@@ -38,7 +38,7 @@ SoundPlayerWidgetAdvancedView::SoundPlayerWidgetAdvancedView(GUI::Window& window
 
     m_playlist_widget = PlaylistWidget::construct();
     m_playlist_widget->set_data_model(playlist().model());
-    m_playlist_widget->set_fixed_width(150);
+    m_playlist_widget->set_preferred_width(150);
 
     m_player_view->set_layout<GUI::VerticalBoxLayout>();
 
diff --git a/Userland/Applications/Spreadsheet/SpreadsheetWidget.cpp b/Userland/Applications/Spreadsheet/SpreadsheetWidget.cpp
index f35fc2a34ec..33fc5a06336 100644
--- a/Userland/Applications/Spreadsheet/SpreadsheetWidget.cpp
+++ b/Userland/Applications/Spreadsheet/SpreadsheetWidget.cpp
@@ -38,7 +38,7 @@ SpreadsheetWidget::SpreadsheetWidget(GUI::Window& parent_window, NonnullRefPtrVe
 
     auto& top_bar = container.add<GUI::Frame>();
     top_bar.set_layout<GUI::HorizontalBoxLayout>().set_spacing(1);
-    top_bar.set_fixed_height(26);
+    top_bar.set_preferred_height(26);
     auto& current_cell_label = top_bar.add<GUI::Label>("");
     current_cell_label.set_fixed_width(50);
 
diff --git a/Userland/Applications/TextEditor/TextEditorWindow.gml b/Userland/Applications/TextEditor/TextEditorWindow.gml
index b8556df9a85..1a1c3b1d85d 100644
--- a/Userland/Applications/TextEditor/TextEditorWindow.gml
+++ b/Userland/Applications/TextEditor/TextEditorWindow.gml
@@ -14,7 +14,7 @@
     }
 
     @GUI::HorizontalSplitter {
-        fixed_resizee: "Second"
+        opportunistic_resizee: "First"
 
         @GUI::TextEditor {
             name: "editor"
diff --git a/Userland/DevTools/HackStudio/HackStudioWidget.cpp b/Userland/DevTools/HackStudio/HackStudioWidget.cpp
index e4a3127501a..cbac2156ccd 100644
--- a/Userland/DevTools/HackStudio/HackStudioWidget.cpp
+++ b/Userland/DevTools/HackStudio/HackStudioWidget.cpp
@@ -94,7 +94,7 @@ HackStudioWidget::HackStudioWidget(String path_to_project)
 
     auto& left_hand_splitter = outer_splitter.add<GUI::VerticalSplitter>();
     left_hand_splitter.layout()->set_spacing(6);
-    left_hand_splitter.set_fixed_width(150);
+    left_hand_splitter.set_preferred_width(150);
     create_project_tab(left_hand_splitter);
     m_project_tree_view_context_menu = create_project_tree_view_context_menu();
 
@@ -980,7 +980,7 @@ NonnullRefPtr<GUI::Action> HackStudioWidget::create_add_terminal_action()
 void HackStudioWidget::reveal_action_tab(GUI::Widget& widget)
 {
     if (m_action_tab_widget->effective_min_size().height().as_int() < 200)
-        m_action_tab_widget->set_fixed_height(200);
+        m_action_tab_widget->set_preferred_height(200);
     m_action_tab_widget->set_active_widget(&widget);
 }
 
@@ -1149,7 +1149,7 @@ void HackStudioWidget::run()
 
 void HackStudioWidget::hide_action_tabs()
 {
-    m_action_tab_widget->set_fixed_height(24);
+    m_action_tab_widget->set_preferred_height(24);
 };
 
 Project& HackStudioWidget::project()
@@ -1300,13 +1300,13 @@ void HackStudioWidget::create_action_tab(GUI::Widget& parent)
 {
     m_action_tab_widget = parent.add<GUI::TabWidget>();
 
-    m_action_tab_widget->set_fixed_height(24);
+    m_action_tab_widget->set_preferred_height(24);
     m_action_tab_widget->on_change = [this](auto&) {
         on_action_tab_change();
 
         static bool first_time = true;
         if (!first_time)
-            m_action_tab_widget->set_fixed_height(200);
+            m_action_tab_widget->set_preferred_height(200);
         first_time = false;
     };
 
diff --git a/Userland/DevTools/Inspector/main.cpp b/Userland/DevTools/Inspector/main.cpp
index bf0fce471c0..cedb75537f5 100644
--- a/Userland/DevTools/Inspector/main.cpp
+++ b/Userland/DevTools/Inspector/main.cpp
@@ -109,7 +109,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     auto& tree_view = splitter.add<GUI::TreeView>();
     tree_view.set_model(remote_process.object_graph_model());
     tree_view.set_activates_on_selection(true);
-    tree_view.set_fixed_width(286);
+    tree_view.set_preferred_width(286);
 
     auto& properties_tree_view = splitter.add<GUI::TreeView>();
     properties_tree_view.set_should_fill_selected_rows(true);