fix S10 code-review findings: WM_CAPTURECHANGED, proportional drag, paintSetup root source, Add Zone dedup, strip-area helpers

src/vst/keyboard_strip.cpp

@@ -106,12 +106,20 @@ int resolveDragNote(const StripLayout& layout, int startNote, int dxPixels) {
     if (dxPixels == 0) return clampNote(startNote);
     const int bandWidth = std::max(0, layout.keys.width());
     if (bandWidth <= 0) return clampNote(startNote);  // zero-width -> no motion
-    // Key width in pixels (>= 1 via the max). Round the delta to the nearest key so a
-    // half-key drag flips at the key centre: add/subtract half a key before the divide.
-    const int keyW = std::max(1, bandWidth / kStripKeyCount);
-    const int half = keyW / 2;
-    const int shift = dxPixels >= 0 ? (dxPixels + half) / keyW
-                                    : -((-dxPixels + half) / keyW);
+    // Proportional shift: same linear mapping as keyAtPoint/keyEdgeToX so click and drag
+    // agree across the full strip, even on non-divisible-by-128 widths. The proportional
+    // key width is (bandWidth / kStripKeyCount) in exact rational arithmetic; rounding to
+    // the nearest key (half-key drag flips at the key centre) is achieved by adding
+    // bandWidth/2 to the absolute pixel delta before dividing — identical to the old
+    // formula except keyWidth is now derived from the same linear map (exact rational)
+    // rather than the truncated-integer bandWidth/128 that caused drift at the far end.
+    const int half = bandWidth / 2;
+    int shift;
+    if (dxPixels > 0) {
+        shift = (dxPixels * kStripKeyCount + half) / bandWidth;
+    } else {
+        shift = -(((-dxPixels) * kStripKeyCount + half) / bandWidth);
+    }
     return clampNote(startNote + shift);
 }