diff --git a/negpy/desktop/view/widgets/prescan_dialog.py b/negpy/desktop/view/widgets/prescan_dialog.py
index 29abbbb1..e35b3157 100644
--- a/negpy/desktop/view/widgets/prescan_dialog.py
+++ b/negpy/desktop/view/widgets/prescan_dialog.py
@@ -18,7 +18,7 @@
 from negpy.desktop.view.widgets.scan_window_label import ScanWindowLabel
 from negpy.desktop.workers.scan_worker import PrescanRequest
 from negpy.infrastructure.scanners.base import ScannerDevice
-from negpy.infrastructure.scanners.params import clamp_scan_area
+from negpy.infrastructure.scanners.params import crop_to_scan_window
 from negpy.infrastructure.scanners.result import ScanResult
 
 
@@ -53,8 +53,10 @@ def __init__(
         super().__init__(parent)
         self._controller = controller
         self._device = device
+        caps = device.capabilities
         # TA / backend space (what ScanParams.window stores).
         self._scan_window: tuple[float, float, float, float] | None = initial_window
+        self._prescan_mirror_x = bool(caps.prescan_mirror_x)
         self._busy = False
 
         self.setWindowTitle("Prescan — set crop")
@@ -152,7 +154,7 @@ def _on_prescan_ready(self, result: object) -> None:
             if default_crop is not None:
                 self._scan_window = default_crop
         if self._scan_window is not None:
-            image_rect = clamp_scan_area(self._scan_window)
+            image_rect = crop_to_scan_window(self._scan_window, mirror_x=self._prescan_mirror_x)
             self._label.set_window(image_rect)
         self._ok_btn.setEnabled(True)
         self._status.setText("Drag the rectangle to set the scan crop")
@@ -177,7 +179,7 @@ def _on_window_changed(self, rect: object) -> None:
         if rect is None:
             self._scan_window = None
             return
-        self._scan_window = clamp_scan_area(tuple(rect))  # type: ignore[arg-type]
+        self._scan_window = crop_to_scan_window(tuple(rect), mirror_x=self._prescan_mirror_x)  # type: ignore[arg-type]
 
     def _on_clear_crop(self) -> None:
         self._scan_window = None
@@ -197,7 +199,7 @@ def accept(self) -> None:
         # Sync from widget in case the last drag did not emit.
         rect = self._label.window()
         if rect is not None:
-            self._scan_window = clamp_scan_area(rect)
+            self._scan_window = crop_to_scan_window(rect, mirror_x=self._prescan_mirror_x)
         self._disconnect_controller()
         super().accept()
 
diff --git a/negpy/infrastructure/scanners/base.py b/negpy/infrastructure/scanners/base.py
index 256089fe..45adb98b 100644
--- a/negpy/infrastructure/scanners/base.py
+++ b/negpy/infrastructure/scanners/base.py
@@ -34,6 +34,8 @@ class ScannerCapabilities:
     #: Low-DPI full-window preview then interactive crop (Plustek SE).
     prescan: bool = False
     prescan_dpi: int = 0
+    #: Left–right mirrored sensor (sensor order flipped in pyopticfilm's assemble()).
+    prescan_mirror_x: bool = False
     prescan_default_crop: tuple[float, float, float, float] | None = None
     multi_exposure: bool = False
     adapter_frame_capacity: int | None = None  # transport capacity bound, not an exposure count
diff --git a/negpy/infrastructure/scanners/params.py b/negpy/infrastructure/scanners/params.py
index 85aa9679..c77ec16f 100644
--- a/negpy/infrastructure/scanners/params.py
+++ b/negpy/infrastructure/scanners/params.py
@@ -50,6 +50,22 @@ def clamp_scan_area(area: ScanArea) -> ScanArea:
     return (x1, y1, x2, y2)
 
 
+def crop_to_scan_window(crop: ScanArea, *, mirror_x: bool) -> ScanArea:
+    """Map Prescan widget coords ↔ TA ``area`` for ``ScanParams.window``.
+
+    Self-inverse when ``mirror_x`` is fixed: image-left is sensor-right on mirrored
+    scanners, so trimming left chrome on the Prescan must crop the opposite TA side.
+
+    Mirrors the pre-#958 behaviour that PR #958 accidentally flattened to a plain
+    clamp, which displaced Prescan crops on mirror_x Plustek devices.
+    """
+    area = clamp_scan_area(crop)
+    if mirror_x:
+        x1, y1, x2, y2 = area
+        return (1.0 - x2, y1, 1.0 - x1, y2)
+    return area
+
+
 def clamp_frame_offset_mm(offset_mm: float, pitch_mm: float) -> float:
     """Effective feed-axis offset, floored at 0 and held short of one frame pitch.
 
diff --git a/negpy/infrastructure/scanners/plustek_backend.py b/negpy/infrastructure/scanners/plustek_backend.py
index e6a3c1e6..d5bbe77a 100644
--- a/negpy/infrastructure/scanners/plustek_backend.py
+++ b/negpy/infrastructure/scanners/plustek_backend.py
@@ -57,6 +57,7 @@ def _caps_for(model: Any) -> ScannerCapabilities:
         autofocus=False,
         prescan=prescan_ready,
         prescan_dpi=PRESCAN_DPI if prescan_ready else 0,
+        prescan_mirror_x=bool(getattr(model, "mirror_x", False)) if prescan_ready else False,
         prescan_default_crop=default_frame_crop_norm(model) if prescan_ready else None,
         multi_exposure=bool(getattr(model, "scan_ready", False) and getattr(model, "exposure_long", None)),
         adapter_frame_capacity=None,
diff --git a/tests/scanners/test_params.py b/tests/scanners/test_params.py
index fd41edb4..b517fcb3 100644
--- a/tests/scanners/test_params.py
+++ b/tests/scanners/test_params.py
@@ -1,7 +1,12 @@
 """Tests for ScanParams dataclass and ScanMode validation."""
 
 import pytest
-from negpy.infrastructure.scanners.params import ScanMode, ScanParams, clamp_scan_area
+from negpy.infrastructure.scanners.params import (
+    ScanMode,
+    ScanParams,
+    clamp_scan_area,
+    crop_to_scan_window,
+)
 from negpy.infrastructure.scanners.base import ScannerCapabilities
 
 
@@ -72,3 +77,19 @@ def test_dpi_range_from_caps(self) -> None:
 class TestPrescanGeometry:
     def test_clamp_scan_area_enforces_positive_extent(self) -> None:
         assert clamp_scan_area((0.5, 0.5, 0.5, 0.5)) == (0.5, 0.5, 0.501, 0.501)
+
+    def test_crop_to_scan_window_reflects_x_when_mirrored(self) -> None:
+        """Mirror_x scanners: display-left is sensor-right, so x must be reflected.
+
+        Regression for PR #958, which flattened this conversion to a plain clamp and
+        displaced Prescan crops on mirror_x Plustek devices (8200i SE / 8100 V2).
+        """
+        assert crop_to_scan_window((0.1, 0.2, 0.4, 0.8), mirror_x=True) == pytest.approx((0.6, 0.2, 0.9, 0.8))
+
+    def test_crop_to_scan_window_passthrough_when_not_mirrored(self) -> None:
+        assert crop_to_scan_window((0.1, 0.2, 0.4, 0.8), mirror_x=False) == pytest.approx((0.1, 0.2, 0.4, 0.8))
+
+    def test_crop_to_scan_window_is_self_inverse(self) -> None:
+        crop = (0.1, 0.2, 0.4, 0.8)
+        reflected = crop_to_scan_window(crop, mirror_x=True)
+        assert crop_to_scan_window(reflected, mirror_x=True) == pytest.approx(crop)
diff --git a/tests/scanners/test_plustek_backend.py b/tests/scanners/test_plustek_backend.py
index 95caf78f..06f9d8c1 100644
--- a/tests/scanners/test_plustek_backend.py
+++ b/tests/scanners/test_plustek_backend.py
@@ -181,6 +181,7 @@ def test_8100_v2_caps_match_pyopticfilm_model(monkeypatch):
     assert caps.ir_channel is False
     assert caps.multi_exposure is True
     assert caps.prescan is True
+    assert caps.prescan_mirror_x is True  # 8100 V2 inherits mirror_x from 8200i SE
     assert 1200 in caps.supported_dpi