diff --git a/src/primitives.c b/src/primitives.c
index 86ae68a..a273277 100644
--- a/src/primitives.c
+++ b/src/primitives.c
@@ -33,9 +33,16 @@ static int AmplitudeToY(ScopeView* view, float amp)
     return view->y + view->height - (int)((amp - view->ampMin) / (view->ampMax - view->ampMin) * view->height);
 }
 
+// Map a signal-space time (0-1 over the WHOLE signal) to a screen X, honoring
+// the visible window. The waveform envelope below already draws only
+// viewStart..viewEnd, so grid lines and the cursor have to use the same mapping
+// or they drift out of register with the trace (and with the spectrogram above)
+// as soon as the user zooms or pans.
 static int TimeToX(ScopeView* view, float t)
 {
-    return view->x + (int)(t * view->width);
+    float span = view->viewEnd - view->viewStart;
+    if (span <= 0.0f) span = 1.0f;
+    return view->x + (int)((t - view->viewStart) / span * view->width);
 }
 
 void DrawScopeView(ScopeView* view, float cursorT)
@@ -56,9 +63,11 @@ void DrawScopeView(ScopeView* view, float cursorT)
     if (view->showGrid) {
         Color gridColor = (Color){ view->gridR, view->gridG, view->gridB, (int)(view->gridAlpha * 255) };
 
-        // Vertical time divisions
+        // Vertical time divisions — ten evenly spaced lines across the VISIBLE
+        // window, so the grid stays put under zoom instead of sliding off.
         for (int i = 0; i <= 10; i++) {
-            int x = TimeToX(view, (float)i / 10.0f);
+            float t = view->viewStart + (float)i / 10.0f * (view->viewEnd - view->viewStart);
+            int x = TimeToX(view, t);
             DrawLineV((Vector2){ x, view->y }, (Vector2){ x, view->y + view->height }, gridColor);
         }
 
@@ -119,7 +128,8 @@ void DrawScopeView(ScopeView* view, float cursorT)
         DrawLine(px + view->x, yTop, px + view->x, yBot, waveColor);
     }
 
-    // Cursor
+    // Cursor. cursorT is signal-space; TimeToX maps it into the visible window,
+    // and the bounds check below drops it when it falls outside the current view.
     if (cursorT >= 0.0f && cursorT <= 1.0f) {
         int cursorX = TimeToX(view, cursorT);
         if (cursorX >= view->x && cursorX <= view->x + view->width) {