diff --git a/src/static/css/thread_toolbar.css b/src/static/css/thread_toolbar.css
new file mode 100644
index 0000000..cd1ac48
--- /dev/null
+++ b/src/static/css/thread_toolbar.css
@@ -0,0 +1,9 @@
+/* Small adjustement to reduce spacing, since it's excessive when adding the toolbar. */
+body.TWPT-threadtoolbar-shown ec-thread .scTailwindThreadThreadcontentreplies-section {
+  padding-top: 0.5rem;
+}
+
+/* Hide reply button when a thread is flattened, since it might not work correctly */
+body.TWPT-flattenthreads-enabled ec-thread sc-tailwind-thread-message-message-list:last-child .scTailwindThreadMessageMessagecardsub-content {
+  display: none;
+}
diff --git a/src/static/css/ui_spacing/shared.css b/src/static/css/ui_spacing/shared.css
index c5b7cdd..2ae4fd8 100644
--- a/src/static/css/ui_spacing/shared.css
+++ b/src/static/css/ui_spacing/shared.css
@@ -27,7 +27,14 @@
   margin-bottom: 0!important;
 }
 
-.scTailwindThreadMessageMessagelistmessage-card {
+/**
+ * When flattening threads, we hide the reply button (in fact the entire blue
+ * footer), so we will reduce the padding moderately.
+ */
+body.TWPT-flattenthreads-enabled .scTailwindThreadMessageMessagelistmessage-card {
+  padding-bottom: 1rem!important;
+}
+body:not(.TWPT-flattenthreads-enabled) .scTailwindThreadMessageMessagelistmessage-card {
   padding-bottom: 0.75rem!important;
 }
 
diff --git a/src/static/options/experiments.html b/src/static/options/experiments.html
index 615d5ec..18fa34d 100644
--- a/src/static/options/experiments.html
+++ b/src/static/options/experiments.html
@@ -16,6 +16,7 @@
         <div class="option"><input type="checkbox" id="workflows"> <label for="workflows" data-i18n="workflows"></label> <button id="manage-workflows" data-i18n="workflows_manage"></button></div>
         <div class="option"><input type="checkbox" id="extrainfo"> <label for="extrainfo" data-i18n="extrainfo"></label></div>
         <div class="option"><input type="checkbox" id="nestedreplies"> <label for="nestedreplies" data-i18n="nestedreplies"></label></div>
+        <div class="option"><input type="checkbox" id="flattenthreads"> <label for="flattenthreads">Flatten threads (feature name TBD)</label></div>
         <div class="actions"><button id="save" data-i18n="save"></button></div>
       </form>
       <div id="save-indicator"></div>