diff --git a/booru_viewer/gui/grid.py b/booru_viewer/gui/grid.py
index dec992d..01fd61e 100644
--- a/booru_viewer/gui/grid.py
+++ b/booru_viewer/gui/grid.py
@@ -364,6 +364,18 @@ class ThumbnailGrid(QScrollArea):
         self.setWidget(self._flow)
         self.setWidgetResizable(True)
         self.setHorizontalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAlwaysOff)
+        # Force the vertical scrollbar to always be visible so its width
+        # is always reserved in the viewport. With the default
+        # ScrollBarAsNeeded, the viewport width drops by ~14-16px when
+        # the scrollbar first appears, and FlowLayout's column count
+        # (computed as `viewport.width() // (THUMB_SIZE + THUMB_SPACING)`)
+        # flips down by 1 if the grid width was sitting within a
+        # scrollbar-width of a column boundary. Visible symptom: the
+        # grid collapses by a column the first time content overflows.
+        # AlwaysOn keeps the scrollbar's width reserved permanently so
+        # the column math is stable across the scrollbar visibility
+        # transition.
+        self.setVerticalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAlwaysOn)
         self._thumbs: list[ThumbnailWidget] = []
         self._selected_index = -1
         self._multi_selected: set[int] = set()