Clear stale hover on capture-loss and Alt-delete; derive gutter test via chromeRects

src/shell/instrument/editor_platform.cpp

@@ -231,6 +231,10 @@ LRESULT CALLBACK ReaSamplerEditor::wndProc(HWND hwnd, UINT msg, WPARAM wParam,
                     self->drag_ = DragKind::kNone;
                     self->dragParamId_ = -1;
                     self->curvePointIndex_ = -1;  // curve-node drag state (peer reset)
+                    // No cursor position is available here to re-resolve hover (unlike
+                    // onMouseUp's release coordinates), so clear rather than leave it naming
+                    // wherever the drag started.
+                    self->hover_ = HoverTarget{};
                     self->invalidate();
                 }
             }