diff --git a/CHANGELOG.md b/CHANGELOG.md
index 614e350..661d8d5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -31,6 +31,12 @@ All notable changes to this project will be documented here. Format: [Keep a Cha
 - **`LogRecord` key collision**: `log.info(..., extra={"name": ...})` raises `KeyError("Attempt to overwrite 'name' in LogRecord")` because `name` is reserved by Python's stdlib logging. Renamed to `template_name`.
 - **React `currentTarget` null in deferred state updaters**: `onChange={(e) => setX((prev) => ({ ...prev, q: e.currentTarget.value }))}` blanked the page on the first user input because `currentTarget` is cleared after the listener bubble ends, before React invokes the updater. Switched all M5 handlers to `e.target.value`, which persists on the synthetic event.
 
+### Fixed (post-M5 UI — modal layout for the test-template editor)
+- **Modal box capped its width at `max-w-2xl` and had no vertical scroll** (`frontend/src/components/ui/Modal.tsx`): opening **+ New test** rendered the 15-column MITRE matrix inside a 672 px frame with no height cap, so the matrix spilled to the right and the form bottom dropped below the viewport — buttons unreachable, no scroll. Added a `size` prop (default `2xl` for back-compat), `max-h-[calc(100vh-2rem)]` + `flex flex-col` on the dialog, and an inner `min-w-0 flex-1 overflow-y-auto` body so the header stays pinned while the form scrolls inside the modal.
+- **MITRE matrix overflow-x failed to scroll inside the modal body** (`frontend/src/components/MitreTagPicker.tsx`): `overflow-x-auto` sat directly on the grid element, but the grid's intrinsic min-width (`15 × minmax(7rem, …)` = 1680 px) prevented it from shrinking below its content, so the grid spilled outside its parent instead of scrolling. Wrapped the grid in a dedicated `overflow-x-auto rounded min-w-0 w-full` scroller and added `min-w-0` to the picker root so the constraint propagates from the modal body. The grid now scrolls horizontally inside the modal.
+- **`grid gap-3` form layout in the test-template modal propagated `min-width: auto`** (`frontend/src/pages/AdminTestsPage.tsx`): each grid item refused to shrink below its widest child, so the picker dragged the form (and the body) past the modal width. Switched the form to `flex flex-col gap-3 min-w-0`, which breaks the propagation while preserving vertical spacing.
+- **Test-template modal now uses `size="7xl"`** and the scenario-template modal `size="3xl"` to match their content density.
+
 ### Fixed (post-M5 review pass — spec-reviewer + code-reviewer)
 - **Filter combinator was OR, not AND** (`backend/app/services/test_templates.py:235`): `?tactic=TA0002&technique=T1059` returned templates matching *either* facet instead of *both*. Pre-fix also pooled all three UUIDs into a shared `IN` list across three columns, theoretically allowing a UUID collision to match across kinds. Refactored to one IN-subquery per facet, ANDed together via repeated `WHERE id IN (...)`.
 - **Concurrent reorder race on `set_scenario_tests`** (`backend/app/services/scenario_templates.py:207`): two parallel reorders on the same scenario could deadlock on the `UNIQUE(scenario_id, position)` constraint under READ COMMITTED. Added a per-scenario `pg_advisory_xact_lock(0x5C3, hash(scenario_id))` mirroring the M4 `/mitre/sync` pattern; different scenarios don't contend.
diff --git a/frontend/src/components/MitreTagPicker.tsx b/frontend/src/components/MitreTagPicker.tsx
index 4a51fef..e6312bc 100644
--- a/frontend/src/components/MitreTagPicker.tsx
+++ b/frontend/src/components/MitreTagPicker.tsx
@@ -83,7 +83,7 @@ export function MitreTagPicker({ value, onChange, className }: MitreTagPickerPro
   }
 
   return (
-    <div className={cn('rounded-lg border border-border bg-bg-card p-4', className)} data-testid="mitre-tag-picker">
+    <div className={cn('rounded-lg border border-border bg-bg-card p-4 min-w-0', className)} data-testid="mitre-tag-picker">
       {/* Selection chips */}
       {value.length > 0 && (
         <div className="mb-3 flex flex-wrap items-center gap-1" data-testid="mitre-selected">
@@ -132,9 +132,15 @@ export function MitreTagPicker({ value, onChange, className }: MitreTagPickerPro
           aria-label="MITRE ATT&CK matrix"
           /* `minmax(7rem, 1fr)` ensures every cell is wide enough for the
            * longest single word in MITRE names (no mid-word breaks), and
-           * stretches to fill the container otherwise. Horizontal scroll only
-           * kicks in on narrow viewports below ~1680px. */
-          className="grid gap-px bg-border rounded overflow-x-auto"
+           * stretches to fill the container otherwise. The wrapper scrolls
+           * horizontally — placing `overflow-x-auto` on the grid itself fails
+           * because the grid's intrinsic min-width (15 × 7rem) prevents it
+           * from shrinking below its parent, so the grid spills out instead
+           * of scrolling. */
+          className="overflow-x-auto rounded min-w-0 w-full"
+        >
+        <div
+          className="grid gap-px bg-border"
           style={{
             gridTemplateColumns: `repeat(${matrix.data.tactics.length}, minmax(7rem, 1fr))`,
           }}
@@ -276,6 +282,7 @@ export function MitreTagPicker({ value, onChange, className }: MitreTagPickerPro
             );
           })}
         </div>
+        </div>
       )}
 
       <p className="mt-3 font-sans text-[11px] text-text-dim">
diff --git a/frontend/src/components/ui/Modal.tsx b/frontend/src/components/ui/Modal.tsx
index 0bdc464..99d4a17 100644
--- a/frontend/src/components/ui/Modal.tsx
+++ b/frontend/src/components/ui/Modal.tsx
@@ -4,6 +4,20 @@ import { Button } from '@/components/ui/Button';
 import { SectionHeader } from '@/components/ui/SectionHeader';
 import { type Accent } from '@/lib/cn';
 
+type ModalSize = 'md' | 'lg' | 'xl' | '2xl' | '3xl' | '4xl' | '5xl' | '6xl' | '7xl';
+
+const SIZE_CLASS: Record<ModalSize, string> = {
+  md: 'max-w-md',
+  lg: 'max-w-lg',
+  xl: 'max-w-xl',
+  '2xl': 'max-w-2xl',
+  '3xl': 'max-w-3xl',
+  '4xl': 'max-w-4xl',
+  '5xl': 'max-w-5xl',
+  '6xl': 'max-w-6xl',
+  '7xl': 'max-w-7xl',
+};
+
 interface ModalProps {
   open: boolean;
   title: string;
@@ -12,14 +26,27 @@ interface ModalProps {
   children: ReactNode;
   /** Optional name to give the dialog role for screen readers / Playwright. */
   testid?: string;
+  /** Max-width preset. Defaults to `2xl` to keep historical behavior. */
+  size?: ModalSize;
 }
 
 /**
  * Centered modal with a backdrop. Closes on Escape and on backdrop click.
  * The accessible name comes from the SectionHeader's `highlight`, so the dialog
  * can be located via `getByRole('dialog', { name: ... })`.
+ *
+ * The dialog caps its height at the viewport and scrolls its body internally,
+ * so tall content (MITRE matrix, long forms) never escapes the viewport.
  */
-export function Modal({ open, title, accent = 'cyan', onClose, children, testid }: ModalProps) {
+export function Modal({
+  open,
+  title,
+  accent = 'cyan',
+  onClose,
+  children,
+  testid,
+  size = '2xl',
+}: ModalProps) {
   const ref = useRef<HTMLDivElement>(null);
 
   useEffect(() => {
@@ -47,15 +74,15 @@ export function Modal({ open, title, accent = 'cyan', onClose, children, testid
         aria-modal="true"
         aria-label={title}
         data-testid={testid}
-        className="w-full max-w-2xl rounded-lg border border-border bg-bg-base p-6 shadow-2xl"
+        className={`flex w-full ${SIZE_CLASS[size]} max-h-[calc(100vh-2rem)] flex-col rounded-lg border border-border bg-bg-base shadow-2xl`}
       >
-        <div className="flex items-start justify-between gap-4">
+        <div className="flex shrink-0 items-start justify-between gap-4 border-b border-border px-6 pt-6 pb-2">
           <SectionHeader prefix="Edit" highlight={title} accent={accent} className="mt-0 mb-4" />
           <Button variant="ghost" onClick={onClose} aria-label="Close dialog">
             ✕
           </Button>
         </div>
-        {children}
+        <div className="min-w-0 flex-1 overflow-y-auto px-6 py-4">{children}</div>
       </div>
     </div>
   );
diff --git a/frontend/src/pages/AdminScenariosPage.tsx b/frontend/src/pages/AdminScenariosPage.tsx
index ed0f18c..8c9d6bd 100644
--- a/frontend/src/pages/AdminScenariosPage.tsx
+++ b/frontend/src/pages/AdminScenariosPage.tsx
@@ -309,6 +309,7 @@ export function AdminScenariosPage() {
         open={isModalOpen}
         title={editing ? `Scenario · ${editing.name}` : 'New scenario template'}
         accent="purple"
+        size="3xl"
         onClose={() => {
           setCreating(false);
           setEditing(null);
diff --git a/frontend/src/pages/AdminTestsPage.tsx b/frontend/src/pages/AdminTestsPage.tsx
index ceb7b04..29f89aa 100644
--- a/frontend/src/pages/AdminTestsPage.tsx
+++ b/frontend/src/pages/AdminTestsPage.tsx
@@ -281,6 +281,7 @@ export function AdminTestsPage() {
         open={isModalOpen}
         title={editing ? `Test · ${editing.name}` : 'New test template'}
         accent="orange"
+        size="7xl"
         onClose={() => {
           setCreating(false);
           setEditing(null);
@@ -288,7 +289,7 @@ export function AdminTestsPage() {
         testid="test-template-modal"
       >
         {error && <Alert accent="red" className="mb-3">{error}</Alert>}
-        <div className="grid gap-3">
+        <div className="flex flex-col gap-3 min-w-0">
           <TextField
             label="Name"
             value={form.name}