diff --git a/openspec/changes/fix-combobox-flex-collapse/.openspec.yaml b/openspec/changes/fix-combobox-flex-collapse/.openspec.yaml
new file mode 100644
index 0000000000..93831bd262
--- /dev/null
+++ b/openspec/changes/fix-combobox-flex-collapse/.openspec.yaml
@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-05-13
diff --git a/openspec/changes/fix-combobox-flex-collapse/proposal.md b/openspec/changes/fix-combobox-flex-collapse/proposal.md
new file mode 100644
index 0000000000..075b551da1
--- /dev/null
+++ b/openspec/changes/fix-combobox-flex-collapse/proposal.md
@@ -0,0 +1,40 @@
+## Why
+
+Combobox shrinks to 1px when placed in a flex container with `align-items: center` (row). The widget becomes unusable — invisible to the user — without any CSS override from the app developer.
+
+## What changes
+
+`packages/pluggableWidgets/combobox-web/src/ui/Combobox.scss` — `.widget-combobox` rule (line 21).
+
+Replace `min-width: min-content` with `min-width: 15ch`.
+
+`min-content` was the initial fix attempt but does not work — it asks children for their minimum width, and children suppress their own minimums (see Root Cause). An explicit `ch`-based value breaks the dependency on children entirely.
+
+## Root cause
+
+The actual DOM chain in the bug scenario:
+
+```
+.row-center  (user's CSS: display:flex, flex-flow:row, align-items:center)
+  └─ .form-group.no-columns  (Mendix-injected: flex-direction:column, no explicit width)
+       └─ .widget-combobox  (flex-grow:1 — grows height in column container, not width)
+            └─ .widget-combobox-input-container  (flex-grow:1, no min-width)
+                 └─ .widget-combobox-selected-items  (min-width:0 — intentional for tag wrapping)
+                      └─ .widget-combobox-input  (max-width:0 unfocused, width:1px multiselect inactive)
+```
+
+The user created `.row-center`. Mendix injected `.form-group.no-columns` as an intermediate wrapper. That wrapper has no explicit width and becomes a column-direction flex container. Inside it, `flex-grow:1` on `.widget-combobox` distributes height, not width. The widget's width is sized by content, and `min-width: min-content` resolves to ~0 because `.widget-combobox-selected-items` has `min-width: 0` (needed for multiselect tag wrapping).
+
+`min-width: 15ch` is an explicit value — it does not ask children, so the children's suppressed minimums are irrelevant.
+
+## Why `ch` not `px`
+
+`ch` scales with font-size and font-family, respecting user accessibility settings and Atlas theme font changes. "15 characters wide" is a semantically honest statement about a text input widget. Override via normal CSS specificity (`.widget-combobox { min-width: 0 }`).
+
+## Impact
+
+Must not break:
+
+- Single-select combobox layout in all container types
+- Multiselect combobox layout (active and inactive states)
+- Atlas UI demo site rendering (already unaffected per ticket)
diff --git a/openspec/changes/fix-combobox-flex-collapse/tests.md b/openspec/changes/fix-combobox-flex-collapse/tests.md
new file mode 100644
index 0000000000..a1a1e9bef8
--- /dev/null
+++ b/openspec/changes/fix-combobox-flex-collapse/tests.md
@@ -0,0 +1,29 @@
+## Tests
+
+- [ ] **Combobox root element has min-width: 15ch in stylesheet**
+    - **Type:** unit
+    - **Given:** Combobox.scss source
+    - **When:** `.widget-combobox` rule is inspected
+    - **Then:** `min-width` is `15ch`
+    - **Status:** needs update — `src/__tests__/ComboboxStyles.spec.ts` currently asserts `min-content`
+
+- [ ] **Single-select combobox renders with visible width in flex container with align-items center**
+    - **Type:** e2e
+    - **Given:** Single-select Combobox on page `/p/combobox-flex-layout` (`.mx-name-comboBoxFlexSingle`)
+    - **When:** Page loads with no interaction
+    - **Then:** Combobox bounding rect width is greater than 10px
+    - **Status:** pending — needs test page in Studio Pro
+
+- [ ] **Multiselect combobox renders with visible width in flex container with align-items center**
+    - **Type:** e2e
+    - **Given:** Multiselect Combobox on page `/p/combobox-flex-layout` (`.mx-name-comboBoxFlexMulti`)
+    - **When:** Page loads with no interaction
+    - **Then:** Combobox bounding rect width is greater than 10px
+    - **Status:** pending — needs test page in Studio Pro
+
+- [x] **Multiselect inactive input still collapses to 1px (intentional behavior preserved)**
+    - **Type:** unit
+    - **Given:** Combobox.scss source
+    - **When:** multiselect inactive rule inspected
+    - **Then:** `.widget-combobox-input` has `width: 1px`
+    - **Status:** done — `src/__tests__/ComboboxStyles.spec.ts`
diff --git a/packages/pluggableWidgets/combobox-web/e2e/Combobox.spec.js b/packages/pluggableWidgets/combobox-web/e2e/Combobox.spec.js
index f9e2729342..64fab6dbf5 100644
--- a/packages/pluggableWidgets/combobox-web/e2e/Combobox.spec.js
+++ b/packages/pluggableWidgets/combobox-web/e2e/Combobox.spec.js
@@ -174,6 +174,28 @@ test.describe("combobox-web", () => {
     });
 });
 
+// WC-3409: flex align-items center collapse fix
+test.describe("flex container layout (WC-3409)", () => {
+    test.beforeEach(async ({ page }) => {
+        await page.goto("/p/combobox-flex-layout");
+        await waitForMendixApp(page);
+    });
+
+    test("single-select combobox has visible width in flex align-items:center container", async ({ page }) => {
+        const comboBox = page.locator(".mx-name-comboBoxFlexSingle");
+        await expect(comboBox).toBeVisible({ timeout: 10000 });
+        const box = await comboBox.boundingBox();
+        expect(box.width).toBeGreaterThan(10);
+    });
+
+    test("multiselect combobox has visible width in flex align-items:center container", async ({ page }) => {
+        const comboBox = page.locator(".mx-name-comboBoxFlexMulti");
+        await expect(comboBox).toBeVisible({ timeout: 10000 });
+        const box = await comboBox.boundingBox();
+        expect(box.width).toBeGreaterThan(10);
+    });
+});
+
 function getOptions(combobox) {
     return combobox.locator(`[role=listbox] [role=option]`);
 }
diff --git a/packages/pluggableWidgets/combobox-web/src/__tests__/ComboboxStyles.spec.ts b/packages/pluggableWidgets/combobox-web/src/__tests__/ComboboxStyles.spec.ts
new file mode 100644
index 0000000000..479034a3b5
--- /dev/null
+++ b/packages/pluggableWidgets/combobox-web/src/__tests__/ComboboxStyles.spec.ts
@@ -0,0 +1,18 @@
+import { readFileSync } from "fs";
+import { join } from "path";
+
+const scss = readFileSync(join(__dirname, "../ui/Combobox.scss"), "utf-8");
+
+describe("Combobox SCSS — flex collapse fix (WC-3409)", () => {
+    test(".widget-combobox has min-width: 15ch", () => {
+        // explicit ch value prevents collapse when Mendix injects .form-group.no-columns
+        // (column flex container with no width), causing min-content to resolve to ~0
+        expect(scss).toMatch(/\.widget-combobox\s*\{[^}]*min-width:\s*15ch/s);
+    });
+
+    test("inactive multiselect input still has width: 1px (cursor-hiding preserved)", () => {
+        expect(scss).toMatch(
+            /widget-combobox-multiselect[\s\S]*?not\(\.widget-combobox-input-container-active\)[\s\S]*?widget-combobox-input[\s\S]*?width:\s*1px/
+        );
+    });
+});
diff --git a/packages/pluggableWidgets/combobox-web/src/ui/Combobox.scss b/packages/pluggableWidgets/combobox-web/src/ui/Combobox.scss
index 60297b500e..a679d5c90a 100644
--- a/packages/pluggableWidgets/combobox-web/src/ui/Combobox.scss
+++ b/packages/pluggableWidgets/combobox-web/src/ui/Combobox.scss
@@ -18,7 +18,7 @@ $cb-skeleton-light: rgba(194, 194, 194, 0.2);
 $cb-skeleton-dark: #d2d2d2;
 
 .widget-combobox {
-    min-width: 0;
+    min-width: 15ch; // prevent collapse in flex containers with no explicit width
     flex-grow: 1;
     position: relative;
     transition: color 150ms ease 0s;