diff --git a/Front/client/src/pages/Home.tsx b/Front/client/src/pages/Home.tsx
index 0fab4b7..8854e93 100644
--- a/Front/client/src/pages/Home.tsx
+++ b/Front/client/src/pages/Home.tsx
@@ -19,6 +19,7 @@ import {
   deriveUserOverridesKey,
   applyPersistedNonFrameOverrides,
   remapPersistedFramesToZoneFrames,
+  validateZoneGeometriesAgainstLayout,
 } from "../utils/slidePlanUtils";
 import {
   parseMdxFile,
@@ -154,6 +155,21 @@ export default function Home() {
         }
         carriedZoneSections[targetPos].push(...zone.section_ids);
       });
+      // IMP-44 (#73) u4 — clear in-memory zone_geometries on layout flip.
+      // The persisted keys were valid for the *prior* preset; carrying them
+      // forward into the new preset would either trip the u1/u2 backend
+      // [override-warning] guards (foreign keys dropped, override_applied
+      // forced back to None) or partially apply on shared keys. Drop them
+      // up-front so the new layout starts from a clean even-split baseline,
+      // and persist a clear sentinel (null) so a subsequent reopen does not
+      // resurrect the stale snapshot from user_overrides.json.
+      const priorGeoms = p.userSelection.overrides.zone_geometries;
+      const hadPriorGeoms =
+        priorGeoms && typeof priorGeoms === "object" && Object.keys(priorGeoms).length > 0;
+      if (p.uploadedFile && hadPriorGeoms) {
+        const key = deriveUserOverridesKey(p.uploadedFile.name);
+        void saveUserOverrides(key, { zone_geometries: null });
+      }
       return {
         ...p,
         userSelection: {
@@ -162,6 +178,7 @@ export default function Home() {
             ...p.userSelection.overrides,
             layout_preset: layoutId,
             zone_sections: carriedZoneSections,
+            zone_geometries: {},
           },
           selectedZoneId: null,
           selectedRegionId: null,
@@ -329,9 +346,28 @@ export default function Home() {
 
       // zone-geometry override — backend 의 build_layout_css 에 전달 (horizontal-2 /
       // vertical-2 만 적용). zone_id (top/bottom/...) → slide-body 내부 0~1 비율.
+      // IMP-44 (#73) u4 — validate against the active layout *before* the
+      // round-trip so foreign-preset keys never reach the backend. Mirrors
+      // the u1/u2 WARN+DROP guards on the frontend side: dropped keys surface
+      // as a toast (so the user knows why their resize "vanished"), and only
+      // the `kept` subset is forwarded. The active layout = the layout the
+      // backend will use, which is `overrides.layout` when the user has set
+      // one, else the default slidePlan preset (mirrors backend resolution).
       const zoneGeometries = state.userSelection.overrides.zone_geometries;
       if (zoneGeometries && Object.keys(zoneGeometries).length > 0) {
-        overrides.zoneGeometries = zoneGeometries;
+        const activeLayout = overrides.layout ?? sourcePlan.layout_preset;
+        const validation = validateZoneGeometriesAgainstLayout(
+          zoneGeometries,
+          activeLayout,
+        );
+        if (Object.keys(validation.dropped).length > 0) {
+          toast.error(
+            `zone_geometries layout-mismatch: dropped ${Object.keys(validation.dropped).join(", ")} (expected ${validation.expectedPositions.join(", ") || "—"}; layout=${activeLayout}).`,
+          );
+        }
+        if (Object.keys(validation.kept).length > 0) {
+          overrides.zoneGeometries = validation.kept;
+        }
       }
 
       // 2026-05-22 — IMP-08 B-3 원래 동작 (sameAsDefault with effectiveSlidePlan) 복귀.
diff --git a/Front/client/src/utils/slidePlanUtils.ts b/Front/client/src/utils/slidePlanUtils.ts
index 79ec225..f2aa653 100644
--- a/Front/client/src/utils/slidePlanUtils.ts
+++ b/Front/client/src/utils/slidePlanUtils.ts
@@ -1,5 +1,6 @@
 import type { UserSelection, SlidePlan, Zone, InternalRegion, LayoutPresetId } from "../types/designAgent";
 import type { UserOverrides } from "../services/userOverridesApi";
+import { computeZonePositions } from "../services/designAgentApi";
 
 // ─── IMP-52 u6 — restore-on-reopen helpers (pure, exported for testing) ────
 // These helpers compose persisted `user_overrides.json` payloads (typed by
@@ -320,3 +321,77 @@ export function getEffectiveLayoutId(slidePlan: SlidePlan | null, selection: Use
   if (selection.overrides.layout_preset) return selection.overrides.layout_preset;
   return slidePlan?.layout_preset || 'single';
 }
+
+// ─── IMP-44 (#73) u3 — zone_geometries layout-mismatch validation ───────────
+// Pure helper paired with the backend [override-warning] guards added in u1
+// (1-D horizontal-2 / vertical-2 branches of `build_layout_css`) and u2 (2-D
+// `_override_to_grid_tracks` call site). Same WARN+DROP / KEEP-known contract,
+// but expressed on the frontend so handleGenerate (u4) can validate against
+// the active layout *before* forwarding and surface a toast on dropped keys.
+//
+// Source of truth for expected positions = `computeZonePositions(layoutPreset)`
+// (designAgentApi.ts), which mirrors backend `layouts.yaml` (positions field).
+// Unknown layout (null / undefined / not in LAYOUT_PRESET_IDS) ⇒ fail-safe
+// drop-all: caller has no contract for projecting geometries onto an unknown
+// preset, so we keep zero keys rather than passing them through verbatim.
+
+export interface ZoneGeometryValue {
+  x: number;
+  y: number;
+  w: number;
+  h: number;
+}
+
+export interface ZoneGeometriesValidationResult {
+  kept: Record<string, ZoneGeometryValue>;
+  dropped: Record<string, ZoneGeometryValue>;
+  expectedPositions: string[];
+  valid: boolean;
+}
+
+export function validateZoneGeometriesAgainstLayout(
+  geoms: Record<string, ZoneGeometryValue> | null | undefined,
+  layoutPreset: LayoutPresetId | string | null | undefined,
+): ZoneGeometriesValidationResult {
+  const kept: Record<string, ZoneGeometryValue> = {};
+  const dropped: Record<string, ZoneGeometryValue> = {};
+  const safeGeoms =
+    geoms && typeof geoms === "object" && !Array.isArray(geoms) ? geoms : null;
+
+  // Unknown-layout fail-safe — drop everything; no expected positions known.
+  if (typeof layoutPreset !== "string" || !LAYOUT_PRESET_IDS.has(layoutPreset)) {
+    if (safeGeoms) {
+      for (const [k, v] of Object.entries(safeGeoms)) {
+        dropped[k] = v;
+      }
+    }
+    return {
+      kept,
+      dropped,
+      expectedPositions: [],
+      valid: Object.keys(dropped).length === 0,
+    };
+  }
+
+  const expectedPositions = computeZonePositions(
+    layoutPreset as LayoutPresetId,
+  ).map((p) => p.name);
+  const expectedSet = new Set(expectedPositions);
+
+  if (safeGeoms) {
+    for (const [k, v] of Object.entries(safeGeoms)) {
+      if (expectedSet.has(k)) {
+        kept[k] = v;
+      } else {
+        dropped[k] = v;
+      }
+    }
+  }
+
+  return {
+    kept,
+    dropped,
+    expectedPositions,
+    valid: Object.keys(dropped).length === 0,
+  };
+}
diff --git a/Front/client/tests/zone_geometries_validation.test.ts b/Front/client/tests/zone_geometries_validation.test.ts
new file mode 100644
index 0000000..c0ece48
--- /dev/null
+++ b/Front/client/tests/zone_geometries_validation.test.ts
@@ -0,0 +1,222 @@
+// IMP-44 (#73) u3 — vitest coverage for `validateZoneGeometriesAgainstLayout`.
+//
+// Pairs with the backend [override-warning] guards added in u1 (1-D
+// horizontal-2 / vertical-2 branches of `build_layout_css`) and u2 (2-D
+// `_override_to_grid_tracks` call site). Same WARN+DROP unknown / KEEP known
+// contract; this helper lets handleGenerate (u4) validate against the active
+// layout before forwarding so the user sees a toast on dropped keys rather
+// than the backend silently even-splitting non-overridden zones with a false
+// `computation=user_override_geometry` signal.
+//
+// Cases (Stage 2 scope-lock):
+//   1) horizontal-2 → vertical-2 mismatch (all keys dropped)
+//   2) passthrough (all keys recognized)
+//   3) partial mix (some kept, some dropped)
+//   4) empty input ({} on a known layout)
+//   5) unknown-layout fail-safe (preset null / undefined / unknown string)
+
+import { describe, it, expect } from "vitest";
+import { validateZoneGeometriesAgainstLayout } from "../src/utils/slidePlanUtils";
+
+const g = (x: number, y: number, w: number, h: number) => ({ x, y, w, h });
+
+describe("validateZoneGeometriesAgainstLayout (IMP-44 u3)", () => {
+  // ── 1. mismatch ──────────────────────────────────────────────────────────
+  it("drops horizontal-2 keys when the active layout is vertical-2", () => {
+    const result = validateZoneGeometriesAgainstLayout(
+      { top: g(0, 0, 1, 0.4), bottom: g(0, 0.4, 1, 0.6) },
+      "vertical-2",
+    );
+    expect(result.kept).toEqual({});
+    expect(result.dropped).toEqual({
+      top: g(0, 0, 1, 0.4),
+      bottom: g(0, 0.4, 1, 0.6),
+    });
+    expect(result.expectedPositions).toEqual(["left", "right"]);
+    expect(result.valid).toBe(false);
+  });
+
+  it("drops vertical-2 keys when the active layout is horizontal-2", () => {
+    const result = validateZoneGeometriesAgainstLayout(
+      { left: g(0, 0, 0.5, 1), right: g(0.5, 0, 0.5, 1) },
+      "horizontal-2",
+    );
+    expect(result.kept).toEqual({});
+    expect(Object.keys(result.dropped).sort()).toEqual(["left", "right"]);
+    expect(result.expectedPositions).toEqual(["top", "bottom"]);
+    expect(result.valid).toBe(false);
+  });
+
+  // ── 2. passthrough ───────────────────────────────────────────────────────
+  it("keeps all keys when every input key is in the active layout positions", () => {
+    const input = {
+      top: g(0, 0, 1, 0.4),
+      bottom: g(0, 0.4, 1, 0.6),
+    };
+    const result = validateZoneGeometriesAgainstLayout(input, "horizontal-2");
+    expect(result.kept).toEqual(input);
+    expect(result.dropped).toEqual({});
+    expect(result.expectedPositions).toEqual(["top", "bottom"]);
+    expect(result.valid).toBe(true);
+  });
+
+  it("passes a single 'primary' key through on the 'single' preset", () => {
+    const result = validateZoneGeometriesAgainstLayout(
+      { primary: g(0, 0, 1, 1) },
+      "single",
+    );
+    expect(result.kept).toEqual({ primary: g(0, 0, 1, 1) });
+    expect(result.dropped).toEqual({});
+    expect(result.expectedPositions).toEqual(["primary"]);
+    expect(result.valid).toBe(true);
+  });
+
+  it("recognizes the 2-D preset positions reported by computeZonePositions (top-1-bottom-2)", () => {
+    const input = {
+      top: g(0, 0, 1, 0.5),
+      "bottom-left": g(0, 0.5, 0.5, 0.5),
+      "bottom-right": g(0.5, 0.5, 0.5, 0.5),
+    };
+    const result = validateZoneGeometriesAgainstLayout(input, "top-1-bottom-2");
+    expect(result.kept).toEqual(input);
+    expect(result.dropped).toEqual({});
+    expect(result.expectedPositions).toEqual([
+      "top",
+      "bottom-left",
+      "bottom-right",
+    ]);
+    expect(result.valid).toBe(true);
+  });
+
+  // ── 3. partial mix ───────────────────────────────────────────────────────
+  it("keeps known keys and drops unknown keys on a partial-mix input", () => {
+    const result = validateZoneGeometriesAgainstLayout(
+      { top: g(0, 0, 1, 0.4), foo: g(0, 0, 1, 1) },
+      "horizontal-2",
+    );
+    expect(result.kept).toEqual({ top: g(0, 0, 1, 0.4) });
+    expect(result.dropped).toEqual({ foo: g(0, 0, 1, 1) });
+    expect(result.expectedPositions).toEqual(["top", "bottom"]);
+    expect(result.valid).toBe(false);
+  });
+
+  it("on a 2-D preset, keeps known 2-D track keys and drops legacy 1-D keys", () => {
+    // Simulates the user resizing under top-1-bottom-2, then flipping to
+    // grid-2x2 — legacy `bottom-left` stays valid; `top` (no longer a 2x2
+    // position) gets dropped.
+    const result = validateZoneGeometriesAgainstLayout(
+      {
+        top: g(0, 0, 1, 0.5),
+        "bottom-left": g(0, 0.5, 0.5, 0.5),
+        "top-left": g(0, 0, 0.5, 0.5),
+      },
+      "grid-2x2",
+    );
+    expect(result.kept).toEqual({
+      "bottom-left": g(0, 0.5, 0.5, 0.5),
+      "top-left": g(0, 0, 0.5, 0.5),
+    });
+    expect(result.dropped).toEqual({ top: g(0, 0, 1, 0.5) });
+    expect(result.expectedPositions).toEqual([
+      "top-left",
+      "top-right",
+      "bottom-left",
+      "bottom-right",
+    ]);
+    expect(result.valid).toBe(false);
+  });
+
+  // ── 4. empty input ───────────────────────────────────────────────────────
+  it("returns empty kept/dropped and valid=true on an empty {} input", () => {
+    const result = validateZoneGeometriesAgainstLayout({}, "horizontal-2");
+    expect(result.kept).toEqual({});
+    expect(result.dropped).toEqual({});
+    expect(result.expectedPositions).toEqual(["top", "bottom"]);
+    expect(result.valid).toBe(true);
+  });
+
+  it("treats null / undefined geoms as empty input (no throw, valid=true on a known layout)", () => {
+    const nullResult = validateZoneGeometriesAgainstLayout(null, "vertical-2");
+    expect(nullResult.kept).toEqual({});
+    expect(nullResult.dropped).toEqual({});
+    expect(nullResult.expectedPositions).toEqual(["left", "right"]);
+    expect(nullResult.valid).toBe(true);
+
+    const undefResult = validateZoneGeometriesAgainstLayout(
+      undefined,
+      "vertical-2",
+    );
+    expect(undefResult.kept).toEqual({});
+    expect(undefResult.dropped).toEqual({});
+    expect(undefResult.expectedPositions).toEqual(["left", "right"]);
+    expect(undefResult.valid).toBe(true);
+  });
+
+  it("ignores array payloads (defensive against hand-edited persisted files)", () => {
+    const result = validateZoneGeometriesAgainstLayout(
+      [] as unknown as Record<string, { x: number; y: number; w: number; h: number }>,
+      "horizontal-2",
+    );
+    expect(result.kept).toEqual({});
+    expect(result.dropped).toEqual({});
+    expect(result.expectedPositions).toEqual(["top", "bottom"]);
+    expect(result.valid).toBe(true);
+  });
+
+  // ── 5. unknown-layout fail-safe ──────────────────────────────────────────
+  it("drops every input key when layout is null (fail-safe)", () => {
+    const result = validateZoneGeometriesAgainstLayout(
+      { top: g(0, 0, 1, 0.4), bottom: g(0, 0.4, 1, 0.6) },
+      null,
+    );
+    expect(result.kept).toEqual({});
+    expect(result.dropped).toEqual({
+      top: g(0, 0, 1, 0.4),
+      bottom: g(0, 0.4, 1, 0.6),
+    });
+    expect(result.expectedPositions).toEqual([]);
+    expect(result.valid).toBe(false);
+  });
+
+  it("drops every input key when layout is undefined (fail-safe)", () => {
+    const result = validateZoneGeometriesAgainstLayout(
+      { primary: g(0, 0, 1, 1) },
+      undefined,
+    );
+    expect(result.kept).toEqual({});
+    expect(result.dropped).toEqual({ primary: g(0, 0, 1, 1) });
+    expect(result.expectedPositions).toEqual([]);
+    expect(result.valid).toBe(false);
+  });
+
+  it("drops every input key when layout is an unknown preset string (fail-safe)", () => {
+    const result = validateZoneGeometriesAgainstLayout(
+      { top: g(0, 0, 1, 0.4) },
+      "rogue-preset" as unknown as string,
+    );
+    expect(result.kept).toEqual({});
+    expect(result.dropped).toEqual({ top: g(0, 0, 1, 0.4) });
+    expect(result.expectedPositions).toEqual([]);
+    expect(result.valid).toBe(false);
+  });
+
+  it("returns empty kept/dropped/expectedPositions when layout is unknown AND geoms is empty", () => {
+    const result = validateZoneGeometriesAgainstLayout({}, null);
+    expect(result.kept).toEqual({});
+    expect(result.dropped).toEqual({});
+    expect(result.expectedPositions).toEqual([]);
+    // No keys to drop ⇒ vacuously valid; handleGenerate (u4) gates the toast
+    // on `Object.keys(dropped).length > 0`, not `valid`, so this is safe.
+    expect(result.valid).toBe(true);
+  });
+
+  // ── purity / mutation safety ─────────────────────────────────────────────
+  it("does not mutate the input geometries object", () => {
+    const input = { top: g(0, 0, 1, 0.4), foo: g(0, 0, 1, 1) };
+    const inputKeysBefore = Object.keys(input).sort();
+    validateZoneGeometriesAgainstLayout(input, "horizontal-2");
+    expect(Object.keys(input).sort()).toEqual(inputKeysBefore);
+    // Sample value still pristine.
+    expect(input.top).toEqual(g(0, 0, 1, 0.4));
+  });
+});
diff --git a/src/phase_z2_pipeline.py b/src/phase_z2_pipeline.py
index fd798b1..daa2f1e 100644
--- a/src/phase_z2_pipeline.py
+++ b/src/phase_z2_pipeline.py
@@ -1923,83 +1923,139 @@ def build_layout_css(layout_preset: str, zones_data: list[dict],
     # ── Step D-ext : user override 처리 ──
     if override_zone_geometries:
         if layout_preset == "horizontal-2":
-            # heights_px override — zone 의 h 비율로 SLIDE_BODY_HEIGHT 분배.
-            # Hot-fix (2026-05-22): partial override = 나머지 공간을 비-override zone 들에
-            # 균등 분배 (drag boundary intent). 이전엔 0.0 fallback → 100/0 깨짐.
-            overridden_h = sum(
-                float(override_zone_geometries[p]["h"])
-                for p in positions if p in override_zone_geometries
+            # IMP-44 u1 — unknown-key guard: drop foreign-preset keys
+            # (예: vertical-2 keys {left,right} sent to horizontal-2), emit
+            # structured warning, keep matching keys. All-unknown → fall
+            # through to default dynamic dispatch (no false override_applied).
+            unknown_keys = sorted(
+                k for k in override_zone_geometries if k not in positions
             )
-            non_overridden = [p for p in positions if p not in override_zone_geometries]
-            per_non = max(0.0, 1.0 - overridden_h) / max(len(non_overridden), 1)
-            ratios = []
-            for pos in positions:
-                geom = override_zone_geometries.get(pos)
-                ratios.append(float(geom["h"]) if geom else per_non)
-            total = sum(ratios)
-            if total > 0:
-                heights_px = [int(round(r / total * SLIDE_BODY_HEIGHT)) for r in ratios]
-                rows = " ".join(f"{h}px" for h in heights_px)
-                return {
-                    "areas": preset["css_areas"],
-                    "cols": preset["css_cols"],
-                    "rows": rows,
-                    "heights_px": heights_px,
-                    "widths_px": [SLIDE_BODY_WIDTH],
-                    "ratios": [round(r / total, 3) for r in ratios],
-                    "width_ratios": [1.0],
-                    "computation": "user_override_geometry",
-                    "dynamic_rows": True,
-                    "dynamic_cols": False,
-                    "raw_zone_layout": {"override_applied": True, "source": override_zone_geometries},
-                }
+            if unknown_keys:
+                print(
+                    f"  [override-warning] layout_preset={layout_preset} "
+                    f"expected_positions={list(positions)} unknown_keys={unknown_keys} "
+                    f"(dropped foreign-preset keys; default split for non-overridden).",
+                    file=sys.stderr,
+                )
+            filtered_overrides = {
+                k: v for k, v in override_zone_geometries.items() if k in positions
+            }
+            if filtered_overrides:
+                # heights_px override — zone 의 h 비율로 SLIDE_BODY_HEIGHT 분배.
+                # Hot-fix (2026-05-22): partial override = 나머지 공간을 비-override zone 들에
+                # 균등 분배 (drag boundary intent). 이전엔 0.0 fallback → 100/0 깨짐.
+                overridden_h = sum(
+                    float(filtered_overrides[p]["h"])
+                    for p in positions if p in filtered_overrides
+                )
+                non_overridden = [p for p in positions if p not in filtered_overrides]
+                per_non = max(0.0, 1.0 - overridden_h) / max(len(non_overridden), 1)
+                ratios = []
+                for pos in positions:
+                    geom = filtered_overrides.get(pos)
+                    ratios.append(float(geom["h"]) if geom else per_non)
+                total = sum(ratios)
+                if total > 0:
+                    heights_px = [int(round(r / total * SLIDE_BODY_HEIGHT)) for r in ratios]
+                    rows = " ".join(f"{h}px" for h in heights_px)
+                    return {
+                        "areas": preset["css_areas"],
+                        "cols": preset["css_cols"],
+                        "rows": rows,
+                        "heights_px": heights_px,
+                        "widths_px": [SLIDE_BODY_WIDTH],
+                        "ratios": [round(r / total, 3) for r in ratios],
+                        "width_ratios": [1.0],
+                        "computation": "user_override_geometry",
+                        "dynamic_rows": True,
+                        "dynamic_cols": False,
+                        "raw_zone_layout": {"override_applied": True, "source": filtered_overrides},
+                    }
         elif layout_preset == "vertical-2":
-            # cols override — zone 의 w 비율로 fr 분배 (legacy: fr-string cols).
-            # PR 1 keeps fr-string cols for legacy preserve; widths_px is
-            # populated in pixels for _compute_per_zone_geometry length contract.
-            # Hot-fix (2026-05-22): partial override = 나머지 공간을 비-override zone 들에
-            # 균등 분배 (drag boundary intent). 이전엔 0.0 fallback → 100/0 깨짐.
-            overridden_w = sum(
-                float(override_zone_geometries[p]["w"])
-                for p in positions if p in override_zone_geometries
+            # IMP-44 u1 — unknown-key guard: drop foreign-preset keys
+            # (예: horizontal-2 keys {top,bottom} sent to vertical-2), emit
+            # structured warning, keep matching keys. All-unknown → fall
+            # through to default dynamic dispatch (no false override_applied).
+            unknown_keys = sorted(
+                k for k in override_zone_geometries if k not in positions
             )
-            non_overridden = [p for p in positions if p not in override_zone_geometries]
-            per_non = max(0.0, 1.0 - overridden_w) / max(len(non_overridden), 1)
-            ratios = []
-            for pos in positions:
-                geom = override_zone_geometries.get(pos)
-                ratios.append(float(geom["w"]) if geom else per_non)
-            total = sum(ratios)
-            if total > 0:
-                cols = " ".join(f"{round(r / total * 100, 2)}fr" for r in ratios)
-                normalized = [r / total for r in ratios]
-                widths_px = [
-                    int(round(rr * (SLIDE_BODY_WIDTH - gap * (len(ratios) - 1))))
-                    for rr in normalized
-                ]
-                diff = (SLIDE_BODY_WIDTH - gap * (len(ratios) - 1)) - sum(widths_px)
-                if diff != 0 and widths_px:
-                    widths_px[-1] += diff
-                return {
-                    "areas": preset["css_areas"],
-                    "cols": cols,
-                    "rows": preset["css_rows"],
-                    "heights_px": [SLIDE_BODY_HEIGHT],
-                    "widths_px": widths_px,
-                    "ratios": [1.0],
-                    "width_ratios": [round(rr, 3) for rr in normalized],
-                    "computation": "user_override_geometry",
-                    "dynamic_rows": False,
-                    "dynamic_cols": True,
-                    "raw_zone_layout": {"override_applied": True, "source": override_zone_geometries},
-                }
+            if unknown_keys:
+                print(
+                    f"  [override-warning] layout_preset={layout_preset} "
+                    f"expected_positions={list(positions)} unknown_keys={unknown_keys} "
+                    f"(dropped foreign-preset keys; default split for non-overridden).",
+                    file=sys.stderr,
+                )
+            filtered_overrides = {
+                k: v for k, v in override_zone_geometries.items() if k in positions
+            }
+            if filtered_overrides:
+                # cols override — zone 의 w 비율로 fr 분배 (legacy: fr-string cols).
+                # PR 1 keeps fr-string cols for legacy preserve; widths_px is
+                # populated in pixels for _compute_per_zone_geometry length contract.
+                # Hot-fix (2026-05-22): partial override = 나머지 공간을 비-override zone 들에
+                # 균등 분배 (drag boundary intent). 이전엔 0.0 fallback → 100/0 깨짐.
+                overridden_w = sum(
+                    float(filtered_overrides[p]["w"])
+                    for p in positions if p in filtered_overrides
+                )
+                non_overridden = [p for p in positions if p not in filtered_overrides]
+                per_non = max(0.0, 1.0 - overridden_w) / max(len(non_overridden), 1)
+                ratios = []
+                for pos in positions:
+                    geom = filtered_overrides.get(pos)
+                    ratios.append(float(geom["w"]) if geom else per_non)
+                total = sum(ratios)
+                if total > 0:
+                    cols = " ".join(f"{round(r / total * 100, 2)}fr" for r in ratios)
+                    normalized = [r / total for r in ratios]
+                    widths_px = [
+                        int(round(rr * (SLIDE_BODY_WIDTH - gap * (len(ratios) - 1))))
+                        for rr in normalized
+                    ]
+                    diff = (SLIDE_BODY_WIDTH - gap * (len(ratios) - 1)) - sum(widths_px)
+                    if diff != 0 and widths_px:
+                        widths_px[-1] += diff
+                    return {
+                        "areas": preset["css_areas"],
+                        "cols": cols,
+                        "rows": preset["css_rows"],
+                        "heights_px": [SLIDE_BODY_HEIGHT],
+                        "widths_px": widths_px,
+                        "ratios": [1.0],
+                        "width_ratios": [round(rr, 3) for rr in normalized],
+                        "computation": "user_override_geometry",
+                        "dynamic_rows": False,
+                        "dynamic_cols": True,
+                        "raw_zone_layout": {"override_applied": True, "source": filtered_overrides},
+                    }
         elif topology in ("T", "inverted-T", "side-T-left", "side-T-right", "2x2"):
             # IMP-09 PR 2 — 2-D override path (T / inverted-T / side-T / 2x2).
             # Degenerate inputs (total_h == 0 or total_w == 0) fall back to
             # _build_grid_dynamic_2d inside the helper.
-            return _override_to_grid_tracks(
-                preset, zones_data, override_zone_geometries, gap=gap
+            #
+            # IMP-44 u2 — unknown-key guard mirrors u1 (1-D): drop foreign-
+            # preset keys (예: vertical-2 keys {left,right} sent to T-preset),
+            # emit structured warning, keep matching keys. All-unknown → fall
+            # through to _build_grid_dynamic_2d default (no false override_applied).
+            unknown_keys = sorted(
+                k for k in override_zone_geometries if k not in positions
             )
+            if unknown_keys:
+                print(
+                    f"  [override-warning] layout_preset={layout_preset} "
+                    f"expected_positions={list(positions)} unknown_keys={unknown_keys} "
+                    f"(dropped foreign-preset keys; default split for non-overridden).",
+                    file=sys.stderr,
+                )
+            filtered_overrides = {
+                k: v for k, v in override_zone_geometries.items() if k in positions
+            }
+            if filtered_overrides:
+                return _override_to_grid_tracks(
+                    preset, zones_data, filtered_overrides, gap=gap
+                )
+            return _build_grid_dynamic_2d(preset, zones_data, gap=gap)
         else:
             # warn-and-fallthrough preserved for remaining presets (single).
             # PR 3 territory.
diff --git a/tests/phase_z2/test_build_layout_css_pr1.py b/tests/phase_z2/test_build_layout_css_pr1.py
index 486865e..a799db7 100644
--- a/tests/phase_z2/test_build_layout_css_pr1.py
+++ b/tests/phase_z2/test_build_layout_css_pr1.py
@@ -156,3 +156,109 @@ def test_top_1_bottom_2_dynamic_2d_populates_geometry():
     assert result["dynamic_cols"] is True
     assert len(result["heights_px"]) == 2   # R rows
     assert len(result["widths_px"]) == 2    # C cols
+
+
+# ────────────────────── IMP-44 u5 regression ──────────────────────
+# Regression coverage for the layout-override unknown-key guard
+# (Stage 1 root-cause #73). Asserts that foreign-preset keys are
+# dropped, structured [override-warning] is emitted, and
+# computation=user_override_geometry is NEVER reported when the
+# kept-key set is empty (no false override_applied=true).
+
+
+def test_imp44_h2_with_v2_keys_emits_warning_and_falls_through(capsys):
+    """horizontal-2 receiving vertical-2 keys (left/right) → all-unknown:
+    drop both, emit warning, fall through to dynamic dispatch.
+    computation must NOT be user_override_geometry."""
+    zones = [_zone("top", 0.6), _zone("bottom", 0.4)]
+    override = {
+        "left": {"x": 0, "y": 0, "w": 0.5, "h": 1.0},
+        "right": {"x": 0.5, "y": 0, "w": 0.5, "h": 1.0},
+    }
+    result = build_layout_css(
+        "horizontal-2", zones, override_zone_geometries=override
+    )
+    captured = capsys.readouterr()
+    assert "[override-warning]" in captured.err
+    assert "layout_preset=horizontal-2" in captured.err
+    assert "unknown_keys=['left', 'right']" in captured.err
+    assert "expected_positions=['top', 'bottom']" in captured.err
+    # All-unknown → no override applied (no silent fallback).
+    assert result["computation"] != "user_override_geometry"
+    raw = result.get("raw_zone_layout") or {}
+    if isinstance(raw, dict):
+        assert raw.get("override_applied") is not True
+
+
+def test_imp44_v2_with_h2_keys_emits_warning_and_falls_through(capsys):
+    """vertical-2 receiving horizontal-2 keys (top/bottom) → all-unknown:
+    drop both, emit warning, fall through to dynamic dispatch.
+    computation must NOT be user_override_geometry."""
+    zones = [_zone("left", 0.5), _zone("right", 0.5)]
+    override = {
+        "top": {"x": 0, "y": 0, "w": 1.0, "h": 0.3},
+        "bottom": {"x": 0, "y": 0.3, "w": 1.0, "h": 0.7},
+    }
+    result = build_layout_css(
+        "vertical-2", zones, override_zone_geometries=override
+    )
+    captured = capsys.readouterr()
+    assert "[override-warning]" in captured.err
+    assert "layout_preset=vertical-2" in captured.err
+    assert "unknown_keys=['bottom', 'top']" in captured.err
+    assert "expected_positions=['left', 'right']" in captured.err
+    assert result["computation"] != "user_override_geometry"
+    raw = result.get("raw_zone_layout") or {}
+    if isinstance(raw, dict):
+        assert raw.get("override_applied") is not True
+
+
+def test_imp44_partial_mix_keeps_known_drops_unknown(capsys):
+    """horizontal-2 receiving {top (known), left (unknown)}: keep top,
+    drop left, emit warning naming only 'left'. override_applied=True
+    must hold and the source must contain only the kept key."""
+    zones = [_zone("top", 0.6), _zone("bottom", 0.4)]
+    override = {
+        "top": {"x": 0, "y": 0, "w": 1.0, "h": 0.3},
+        "left": {"x": 0, "y": 0, "w": 0.5, "h": 1.0},
+    }
+    result = build_layout_css(
+        "horizontal-2", zones, override_zone_geometries=override
+    )
+    captured = capsys.readouterr()
+    assert "[override-warning]" in captured.err
+    assert "unknown_keys=['left']" in captured.err
+    # Known key applied → user_override_geometry computation.
+    assert result["computation"] == "user_override_geometry"
+    raw = result["raw_zone_layout"]
+    assert raw["override_applied"] is True
+    assert set(raw["source"].keys()) == {"top"}
+    # Sanity: top ratio (0.3) drives heights_px[0] < heights_px[1].
+    assert result["heights_px"][0] < result["heights_px"][1]
+
+
+def test_imp44_2d_preset_with_h2_keys_emits_warning_and_falls_through(capsys):
+    """2-D preset (top-1-bottom-2) receiving horizontal-2 keys
+    (top/bottom): all-unknown vs T positions
+    {top, bottom-left, bottom-right} → drop all, emit warning,
+    fall through to 2-D dynamic dispatch."""
+    zones = [
+        _zone("top", 0.5),
+        _zone("bottom-left", 0.25),
+        _zone("bottom-right", 0.25),
+    ]
+    override = {
+        "bottom": {"x": 0, "y": 0.3, "w": 1.0, "h": 0.7},
+    }
+    result = build_layout_css(
+        "top-1-bottom-2", zones, override_zone_geometries=override
+    )
+    captured = capsys.readouterr()
+    assert "[override-warning]" in captured.err
+    assert "layout_preset=top-1-bottom-2" in captured.err
+    assert "unknown_keys=['bottom']" in captured.err
+    # All-unknown → 2-D dynamic fallback (not user_override_geometry).
+    assert result["computation"] == "2d_dynamic_aggregated"
+    raw = result.get("raw_zone_layout") or {}
+    if isinstance(raw, dict):
+        assert raw.get("override_applied") is not True