Fix stale hover latch on drag release, tooltip anchor, border off-by-one; pin gutter golden test, drop DPI framing

src/core/instrument/ui/keyboard_strip.h

@@ -1,9 +1,7 @@
 // keyboard_strip.h — piano-keyboard geometry for the editor's root strip: per-class key
 // rects, hit-test, the root marker, and the note name a hovered key reports.
 //
-// Same-class keys are one integer width by construction. An arbitrary band width is not
-// divisible by the 75 white keys, so the residue lands in symmetric end margins — uniform
-// key widths and gap-free edge-to-edge tiling cannot both hold, and uniformity wins.
+// See src/core/instrument/CLAUDE.md for the uniform-width-vs-edge-to-edge-tiling tradeoff.
 
 #pragma once
 
@@ -21,7 +19,8 @@ inline constexpr int kStripKeyCount = 128;
 inline constexpr int kStripWhiteKeyCount = 75;
 
 struct StripLayout {
-    Rect band;   // the surface handed in, edge to edge
+    Rect band;   // the surface handed in, edge to edge (no src/ consumer; kept as the tests'
+                 // reference bound instead of recomputing Rect::ltrb(0, 0, w, h) at each call site)
     Rect keys;   // the tiled key area, kStripWhiteKeyCount * whiteWidth, centred in band
     int whiteWidth = 0;
     int blackWidth = 0;