feat: overview scrollbar on timeline when zoomed in

Thin 8px scrollbar appears above the ruler when the timeline is zoomed.
Shows a draggable thumb representing the current view window. Click
outside the thumb to jump, drag the thumb to pan. Ruler and track
shift down to make room. Scrollbar hidden when not zoomed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.py

@@ -1672,6 +1672,7 @@ class TimelineWidget(QWidget):
     # (index, new_start, new_end, old_start, old_end)
     scan_region_resized = pyqtSignal(int, float, float, float, float)
 
+    _SCROLLBAR_H = 8  # pixels reserved for the overview scrollbar
     _RULER_H = 22     # pixels reserved for the time ruler
     _HANDLE_H = 8     # height of the playhead triangle
     _EDGE_PX = 3      # pixel tolerance for edge hit detection
@@ -1701,6 +1702,9 @@ class TimelineWidget(QWidget):
         self._pan_active = False
         self._pan_start_x = 0.0
         self._pan_start_view = 0.0
+        # Scrollbar drag state
+        self._sb_drag = False
+        self._sb_drag_offset = 0.0
 
         # Waveform data (numpy array of 0-1 peak values, or None)
         self._waveform = None
@@ -1880,11 +1884,22 @@ class TimelineWidget(QWidget):
         p.setRenderHint(QPainter.RenderHint.Antialiasing, False)
         try:
             w, h = self.width(), self.height()
-            rh = self._RULER_H
+            zoomed = self._view_span > 0 and self._duration > 0
+            sb_h = self._SCROLLBAR_H if zoomed else 0
+            rh = sb_h + self._RULER_H
             th = h - rh          # track height
 
+            # ── scrollbar (overview minimap) ──────────────────────────────
+            if zoomed:
+                p.fillRect(0, 0, w, sb_h, QColor(18, 18, 18))
+                thumb_w = max(12, int(self._view_span / self._duration * w))
+                thumb_x = int(self._view_start / self._duration * w)
+                p.fillRect(thumb_x, 1, thumb_w, sb_h - 2, QColor(90, 90, 90))
+                p.setPen(QPen(QColor(55, 55, 55)))
+                p.drawLine(0, sb_h - 1, w, sb_h - 1)
+
             # ── backgrounds ──────────────────────────────────────────────
-            p.fillRect(0, 0, w, rh, QColor(22, 22, 22))        # ruler bg
+            p.fillRect(0, sb_h, w, self._RULER_H, QColor(22, 22, 22))  # ruler bg
             p.fillRect(0, rh, w, th, QColor(32, 32, 32))               # track bg
 
             # subtle track lane (slightly raised strip in the middle)
@@ -2114,6 +2129,23 @@ class TimelineWidget(QWidget):
 
     def mousePressEvent(self, event):
         x = event.position().x()
+        y = event.position().y()
+        # Scrollbar drag
+        if event.button() == Qt.MouseButton.LeftButton and self._view_span > 0 and y < self._SCROLLBAR_H:
+            w = self.width()
+            thumb_w = max(12, int(self._view_span / self._duration * w))
+            thumb_x = int(self._view_start / self._duration * w)
+            if thumb_x <= x <= thumb_x + thumb_w:
+                self._sb_drag = True
+                self._sb_drag_offset = x - thumb_x
+            else:
+                center_t = x / w * self._duration - self._view_span / 2
+                self._view_start = center_t
+                self._clamp_view()
+                self._sb_drag = True
+                self._sb_drag_offset = thumb_w / 2
+                self.update()
+            return
         # Middle-mouse drag pans the view window.
         if event.button() == Qt.MouseButton.MiddleButton and self._view_span > 0:
             self._pan_active = True
@@ -2138,6 +2170,8 @@ class TimelineWidget(QWidget):
     def mouseDoubleClickEvent(self, event):
         from PyQt6.QtCore import Qt as _Qt
         if event.button() == _Qt.MouseButton.LeftButton:
+            if self._view_span > 0 and event.position().y() < self._SCROLLBAR_H:
+                return
             x = event.position().x()
             for (t, _num, output_path, _span) in self._markers:
                 if abs(x - self._time_to_x(t)) <= 10:
@@ -2153,6 +2187,14 @@ class TimelineWidget(QWidget):
         x = event.position().x()
         w = self.width()
 
+        # Active scrollbar drag
+        if self._sb_drag and event.buttons() & Qt.MouseButton.LeftButton:
+            new_x = x - self._sb_drag_offset
+            self._view_start = new_x / max(w, 1) * self._duration
+            self._clamp_view()
+            self.update()
+            return
+
         # Active middle-mouse pan
         if self._pan_active and event.buttons() & Qt.MouseButton.MiddleButton:
             dx = x - self._pan_start_x
@@ -2212,6 +2254,9 @@ class TimelineWidget(QWidget):
             self.cursor_changed.emit(self._cursor)
 
     def mouseReleaseEvent(self, event):
+        if self._sb_drag and event.button() == Qt.MouseButton.LeftButton:
+            self._sb_drag = False
+            return
         if self._pan_active and event.button() == Qt.MouseButton.MiddleButton:
             self._pan_active = False
             self.unsetCursor()