Move Viewport + _DRIFT_TOLERANCE from preview.py to popout/viewport.py (no behavior change)

Step 2 of the gui/app.py + gui/preview.py structural refactor. Pure
move: the popout viewport NamedTuple and the drift-tolerance constant
are now in their own module under popout/. preview.py grows another
re-export shim line so FullscreenPreview's method bodies (which
reference Viewport and _DRIFT_TOLERANCE by bare name) keep working
unchanged. Shim removed in commit 14. See docs/REFACTOR_PLAN.md.

booru_viewer/gui/popout/viewport.py

@@ -0,0 +1,36 @@
+"""Popout viewport math: persistent intent + drift tolerance."""
+
+from __future__ import annotations
+
+from typing import NamedTuple
+
+
+class Viewport(NamedTuple):
+    """Where and how large the user wants popout content to appear.
+
+    Three numbers, no aspect. Aspect is a property of the currently-
+    displayed post and is recomputed from actual content on every
+    navigation. The viewport stays put across navigations; the window
+    rect is a derived projection (Viewport, content_aspect) → (x,y,w,h).
+
+    `long_side` is the binding edge length: for landscape it becomes
+    width, for portrait it becomes height. Symmetric across the two
+    orientations, which is the property that breaks the
+    width-anchor ratchet that the previous `_fit_to_content` had.
+    """
+    center_x: float
+    center_y: float
+    long_side: float
+
+
+# Maximum drift between our last-dispatched window rect and the current
+# Hyprland-reported rect that we still treat as "no user action happened."
+# Anything within this tolerance is absorbed (Hyprland gap rounding,
+# subpixel accumulation, decoration accounting). Anything beyond it is
+# treated as "the user dragged or resized the window externally" and the
+# persistent viewport gets updated from current state.
+#
+# 2px is small enough not to false-positive on real user drags (which
+# are always tens of pixels minimum) and large enough to absorb the
+# 1-2px per-nav drift that compounds across many navigations.
+_DRIFT_TOLERANCE = 2

booru_viewer/gui/preview.py

@@ -18,37 +18,6 @@ import mpv as mpvlib
 _log = logging.getLogger("booru")
 
 
-class Viewport(NamedTuple):
-    """Where and how large the user wants popout content to appear.
-
-    Three numbers, no aspect. Aspect is a property of the currently-
-    displayed post and is recomputed from actual content on every
-    navigation. The viewport stays put across navigations; the window
-    rect is a derived projection (Viewport, content_aspect) → (x,y,w,h).
-
-    `long_side` is the binding edge length: for landscape it becomes
-    width, for portrait it becomes height. Symmetric across the two
-    orientations, which is the property that breaks the
-    width-anchor ratchet that the previous `_fit_to_content` had.
-    """
-    center_x: float
-    center_y: float
-    long_side: float
-
-
-# Maximum drift between our last-dispatched window rect and the current
-# Hyprland-reported rect that we still treat as "no user action happened."
-# Anything within this tolerance is absorbed (Hyprland gap rounding,
-# subpixel accumulation, decoration accounting). Anything beyond it is
-# treated as "the user dragged or resized the window externally" and the
-# persistent viewport gets updated from current state.
-#
-# 2px is small enough not to false-positive on real user drags (which
-# are always tens of pixels minimum) and large enough to absorb the
-# 1-2px per-nav drift that compounds across many navigations.
-_DRIFT_TOLERANCE = 2
-
-
 ## Overlay styling for the popout's translucent toolbar / controls bar
 ## now lives in the bundled themes (themes/*.qss). The widgets get their
 ## object names set in code (FullscreenPreview / VideoPlayer) so theme QSS
@@ -2269,3 +2238,4 @@ class ImagePreview(QWidget):
 
 # -- Refactor compatibility shims (deleted in commit 14) --
 from .media.constants import VIDEO_EXTENSIONS, _is_video  # re-export for refactor compat
+from .popout.viewport import Viewport, _DRIFT_TOLERANCE  # re-export for refactor compat