diff --git a/web_dialog_size/README.rst b/web_dialog_size/README.rst
index 2da3b071f196..646d735f781b 100644
--- a/web_dialog_size/README.rst
+++ b/web_dialog_size/README.rst
@@ -57,6 +57,10 @@ Known issues / Roadmap
 ======================
 
 - Allow setting default dialog size per user.
+- Some dialogs, like the one opened by *Create and edit...*, also show
+  core's ``onExpand`` button next to this module's one. They do
+  different things (core's opens the full form view), but two similar
+  icons are displayed. Unify them or make them easier to tell apart.
 
 Bug Tracker
 ===========
diff --git a/web_dialog_size/readme/ROADMAP.md b/web_dialog_size/readme/ROADMAP.md
index df8c2291b925..643255b0492c 100644
--- a/web_dialog_size/readme/ROADMAP.md
+++ b/web_dialog_size/readme/ROADMAP.md
@@ -1 +1,5 @@
 - Allow setting default dialog size per user.
+- Some dialogs, like the one opened by *Create and edit...*, also show core's
+  `onExpand` button next to this module's one. They do different things (core's
+  opens the full form view), but two similar icons are displayed. Unify them or
+  make them easier to tell apart.
diff --git a/web_dialog_size/static/description/index.html b/web_dialog_size/static/description/index.html
index 2f1d4b6ee284..bc026032a148 100644
--- a/web_dialog_size/static/description/index.html
+++ b/web_dialog_size/static/description/index.html
@@ -411,6 +411,10 @@ <h2><a class="toc-backref" href="#toc-entry-1">Configuration</a></h2>
 <h2><a class="toc-backref" href="#toc-entry-2">Known issues / Roadmap</a></h2>
 <ul class="simple">
 <li>Allow setting default dialog size per user.</li>
+<li>Some dialogs, like the one opened by <em>Create and edit…</em>, also show
+core’s <tt class="docutils literal">onExpand</tt> button next to this module’s one. They do
+different things (core’s opens the full form view), but two similar
+icons are displayed. Unify them or make them easier to tell apart.</li>
 </ul>
 </div>
 <div class="section" id="bug-tracker">
diff --git a/web_dialog_size/static/src/js/web_dialog_size.esm.js b/web_dialog_size/static/src/js/web_dialog_size.esm.js
index c7fedfed25b2..e6c65abde345 100644
--- a/web_dialog_size/static/src/js/web_dialog_size.esm.js
+++ b/web_dialog_size/static/src/js/web_dialog_size.esm.js
@@ -55,7 +55,17 @@ patch(Dialog.prototype, {
     },
 
     get size() {
-        return this.maximized() ? "dialog_full_screen" : super.size;
+        return this.maximized() ? "fs" : super.size;
+    },
+});
+
+patch(ActionDialog.prototype, {
+    setup() {
+        super.setup();
+        // Core (mail) adds its own expand button to some action dialogs, with its
+        // own state. This module already provides one for every dialog, so hide it.
+        // Defined on the instance so it wins whatever the patch order is.
+        Object.defineProperty(this, "canExpand", {get: () => false});
     },
 });
 
diff --git a/web_dialog_size/static/src/scss/web_dialog_size.scss b/web_dialog_size/static/src/scss/web_dialog_size.scss
index 44e2066f1aa2..791f6b2e2bf0 100644
--- a/web_dialog_size/static/src/scss/web_dialog_size.scss
+++ b/web_dialog_size/static/src/scss/web_dialog_size.scss
@@ -1,11 +1,4 @@
 .modal {
-    .modal-dialog_full_screen {
-        @include media-breakpoint-up(sm) {
-            max-width: 100%;
-            width: calc(100% - 50px);
-        }
-    }
-
     .dialog_button_restore,
     .dialog_button_extend {
         margin-left: auto;
diff --git a/web_dialog_size/static/tests/web_dialog_size.test.js b/web_dialog_size/static/tests/web_dialog_size.test.js
index fd72ad0ddc6b..0c029dbd5421 100644
--- a/web_dialog_size/static/tests/web_dialog_size.test.js
+++ b/web_dialog_size/static/tests/web_dialog_size.test.js
@@ -26,7 +26,7 @@ test("the user can maximize and restore a dialog", async () => {
     await mountWithCleanup(Parent);
     expect(".modal-dialog.modal-md").toHaveCount(1);
     await contains(".dialog_button_extend").click();
-    expect(".modal-dialog.modal-dialog_full_screen").toHaveCount(1);
+    expect(".modal-dialog.modal-fs").toHaveCount(1);
     expect(browser.localStorage.getItem("odoo.web_dialog_size.value")).toBe("true");
     await contains(".dialog_button_restore").click();
     expect(".modal-dialog.modal-md").toHaveCount(1);
@@ -37,7 +37,7 @@ test("a stored preference opens the dialog maximized", async () => {
     browser.localStorage.setItem("odoo.web_dialog_size.value", "true");
     assignDialogTestEnv();
     await mountWithCleanup(Parent);
-    expect(".modal-dialog.modal-dialog_full_screen").toHaveCount(1);
+    expect(".modal-dialog.modal-fs").toHaveCount(1);
 });
 
 test("the server default maximizes dialogs without a stored preference", async () => {
@@ -47,5 +47,5 @@ test("the server default maximizes dialogs without a stored preference", async (
     assignDialogTestEnv();
     await mountWithCleanup(Parent);
     await animationFrame();
-    expect(".modal-dialog.modal-dialog_full_screen").toHaveCount(1);
+    expect(".modal-dialog.modal-fs").toHaveCount(1);
 });