keep the selection panel half a tile off what it describes

The panel had one distance for the view edges, the widgets it steps around,
and the selection alike, so it stood eight pixels from a building and touched
it outright along the top. The gap from the selection is now its own value,
half a tile, and it is horizontal only -- the top edges stay level.

It is sampled where the tile size is known, in the same moment as the anchor
rectangle, and travels with it: a rectangle frozen in one moment has no
meaningful distance to a tile size measured in another. chooseSide now takes
the gap in place of the margin, the band having already taken the margin off.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ne3mejABZoLWKLh8fgpM3x

src/lib/core/FloatingPanelPlacement.h

@@ -10,6 +10,12 @@
 // owner places them in one ordered pass, each into the space the earlier ones left free,
 // and these are the rules they place themselves by. Pure geometry -- no widget is
 // involved, which is what lets the rules be tested without a display.
+//
+// Two distances run through these rules and are deliberately different (see
+// REQ-UI-SELECTION-PANEL). marginPx is the edge margin: what a widget keeps from the
+// view's edges and from the widgets it steps around. selectionGapPx is the gap the
+// selection panel keeps from the selection it describes -- half a tile, which is the
+// wider of the two, so the panel stands clear of the objects rather than touching them.
 
 // The lowest bottom edge available to a widget occupying the horizontal span
 // [leftPx, rightPx] inside band: the band's own bottom, or marginPx above the topmost
@@ -29,19 +35,21 @@ enum class PanelSide
 // The side a panel widthPx wide takes beside anchorRect: the right of it where it fits
 // within band, otherwise the left, and where it fits on neither, whichever side leaves
 // more room -- the one case in which the panel ends up over the selection
-// (REQ-UI-SELECTION-PANEL). Decided once when the selection starts and kept for as long
-// as it lasts, so a card that grows later never flips the panel across the object.
+// (REQ-UI-SELECTION-PANEL). The room a side offers is what is left of it once the panel's
+// gap from the selection is taken off. Decided once when the selection starts and kept for
+// as long as it lasts, so a card that grows later never flips the panel across the object.
 PanelSide chooseSide(const QRect& band, const QRect& anchorRect, int widthPx,
-                     int marginPx);
+                     int selectionGapPx);
 
 // Where a panel of wantedSize stands beside anchorRect on the given side: separated from
-// it by marginPx and growing away from it, its top edge on the anchor's top edge, pushed
-// inside band and above whatever occupies it. The returned height is short of
-// wantedSize's when there was not enough room, which is the caller's cue to scroll its
-// content (REQ-UI-SELECTION-PANEL).
+// it by selectionGapPx and growing away from it, its top edge on the anchor's top edge,
+// pushed inside band and above whatever occupies it. The gap is horizontal only -- the
+// panel's top sits level with the anchor's, however wide the gap. The returned height is
+// short of wantedSize's when there was not enough room, which is the caller's cue to
+// scroll its content (REQ-UI-SELECTION-PANEL).
 QRect placeBesideAnchor(const QRect& band, const QRect& anchorRect, PanelSide side,
                         QSize wantedSize, const std::vector<QRect>& occupiedRects,
-                        int marginPx);
+                        int selectionGapPx, int marginPx);
 
 // Where a panel of wantedSize stands once the player has dragged it to desiredTopLeftPx:
 // at that point, by the same rules that place it beside a selection -- pushed inside band,