diff --git a/src/GleapFrameManager.js b/src/GleapFrameManager.js
index c2275543..cbf6b6a9 100644
--- a/src/GleapFrameManager.js
+++ b/src/GleapFrameManager.js
@@ -22,7 +22,7 @@ import GleapAgentToolManager from './GleapAgentToolManager';
 import { getCaptureCapabilities } from './GleapCaptureSettings';
 import { forwardSurveyAnalyticsEvent } from './GleapSurveyAnalyticsForwarder';
 import { bootstrapGleapFrame, loadFromGleapCache, runFunctionWhenDomIsReady, saveToGleapCache } from './GleapHelper';
-import { widgetLoaderMarkup, widgetMaxHeight } from './UI';
+import { widgetExpandMinWidth, widgetLoaderMarkup, widgetMaxHeight } from './UI';
 
 export default class GleapFrameManager {
   frameUrl = 'https://messenger-app.gleap.io';
@@ -429,14 +429,15 @@ export default class GleapFrameManager {
     }
   }
 
-  // The widget is full screen at <= 450px and the expanded CSS only applies from
-  // 451px (see UI.js). Test the same query as that CSS so a fractional (zoomed)
-  // viewport between the two never counts as expandable.
-  isMobileViewport() {
+  // The expanded CSS only applies from widgetExpandMinWidth (see UI.js); narrower
+  // viewports (mobile full screen, or a zoomed-out phone just past it) keep the
+  // default size. Test the same query as that CSS so a fractional (zoomed)
+  // viewport never counts as expandable while the CSS does not apply.
+  hasRoomToExpand() {
     try {
-      return typeof window.matchMedia === 'function' && !window.matchMedia('(min-width: 451px)').matches;
+      return typeof window.matchMedia !== 'function' || window.matchMedia(`(min-width: ${widgetExpandMinWidth}px)`).matches;
     } catch (e) {
-      return false;
+      return true;
     }
   }
 
@@ -445,7 +446,7 @@ export default class GleapFrameManager {
     if (flowConfig && flowConfig.hideExpandButton) {
       return false;
     }
-    return !this.isSurvey() && !this.isMobileViewport();
+    return !this.isSurvey() && this.hasRoomToExpand();
   }
 
   getWidgetExpandedPreference() {
diff --git a/src/GleapFrameManager.test.js b/src/GleapFrameManager.test.js
index c012c8e1..dc332727 100644
--- a/src/GleapFrameManager.test.js
+++ b/src/GleapFrameManager.test.js
@@ -45,6 +45,7 @@ jest.mock('./GleapHelper', () => ({
 jest.mock('./UI', () => ({
   widgetLoaderMarkup: jest.fn(() => ''),
   widgetMaxHeight: 700,
+  widgetExpandMinWidth: 768,
 }));
 
 const EXPANDED_CLASS = 'gleap-frame-container--expanded';
@@ -178,6 +179,26 @@ describe('GleapFrameManager expand/collapse window', () => {
     expect(fm.gleapFrameContainer.classList.contains(EXPANDED_CLASS)).toBe(false);
   });
 
+  it.each([571, 767.5])('is not expandable on a %spx viewport too narrow for the expanded widget', (width) => {
+    // E.g. a phone with the page zoomed out: past the mobile breakpoint, so the
+    // floating widget shows, but an expanded one would cover nearly the whole page.
+    loadFromGleapCache.mockImplementation((key) => (key === 'widget-expanded' ? true : null));
+    viewportWidth = width;
+    const fm = setup();
+    fm.updateFrameStyle();
+
+    expect(fm.canExpandWidget()).toBe(false);
+    expect(fm.isWidgetExpanded()).toBe(false);
+    expect(fm.gleapFrameContainer.classList.contains(EXPANDED_CLASS)).toBe(false);
+  });
+
+  it('is expandable from the expand breakpoint on', () => {
+    viewportWidth = 768;
+    const fm = setup();
+
+    expect(fm.canExpandWidget()).toBe(true);
+  });
+
   it('collapses and hides the button when a config update turns the setting off', () => {
     const fm = setup();
     fm.setWidgetExpanded(true);
@@ -224,7 +245,7 @@ describe('GleapFrameManager expand/collapse window', () => {
     expect(sentSizeUpdates().pop().data).toEqual({ expandable: true, expanded: false });
   });
 
-  it('applies a stored expanded choice when the window widens past the mobile breakpoint', () => {
+  it('applies a stored expanded choice when the window widens past the expand breakpoint', () => {
     loadFromGleapCache.mockImplementation((key) => (key === 'widget-expanded' ? true : null));
     isMobile = true;
     const fm = setup();
diff --git a/src/UI.js b/src/UI.js
index ea01b9b2..74df2b1e 100644
--- a/src/UI.js
+++ b/src/UI.js
@@ -29,6 +29,11 @@ export const calculateContrast = (hex) => {
 
 export const widgetMaxHeight = 700;
 
+// Narrowest viewport that leaves room around the 690px expanded widget. Below it the
+// expanded widget would cover nearly the whole page (e.g. a phone with the page zoomed
+// out just past the 450px mobile breakpoint), so the widget keeps its default size.
+export const widgetExpandMinWidth = 768;
+
 // Builds the loading placeholder shown while the messenger iframe boots.
 // Mirrors the Messenger-App home backgrounds (BG.svg / BGclassic.svg /
 // BGclassicnofade.svg) so the hand-off from loader to app is seamless.
@@ -806,9 +811,9 @@ export const injectStyledCSS = (
     }
 
     /* Expanded by the end user (expand/collapse button in the messenger header).
-       Below 451px the widget is full screen anyway. The inner max-height must beat
-       the inline 700px set by GleapFrameManager.setAppMode. */
-    @media only screen and (min-width: 451px) {
+       Only on viewports with room for it (see widgetExpandMinWidth). The inner
+       max-height must beat the inline 700px set by GleapFrameManager.setAppMode. */
+    @media only screen and (min-width: ${widgetExpandMinWidth}px) {
       .gleap-frame-container--expanded {
         max-width: 690px !important;
       }