Empty-space click on hunk bar recenters viewport on cursor (minimap-style)

Clicking empty bar space — anywhere that's neither a hunk marker nor
the viewport band — now snaps the band's center to the cursor and
enters a sticky-thumb drag in the same gesture. A quick click jumps
the editor to that fraction of the file; press-and-drag continues
scrubbing without releasing.

Implemented by extending the existing drag path: in
OnMouseLeftButtonDown's empty branch (which previously bubbled), set
_bandDragOffsetFromTop = bandHeight/2 so the cursor maps to the band's
center, transition directly to BarInteractionState.Dragging (skipping
PendingClick — we want the scroll to fire on MouseDown, not require a
MouseMove), and emit one EmitDragScroll. All downstream behaviour
(ghost-band paint, scroll request, MouseUp ending the drag without a
JumpToHunk) flows from the existing Dragging state. No new public
surface; just the new empty-space branch.

If no band exists yet (no file loaded / pre-layout) the click bubbles
as before — nothing meaningful to center on.

AI-Local-Session: b53d5fe5-38cc-41dd-b987-3def5ef3f0d3
AI-Cloud-Session: 744dd836-01e5-48af-91db-daa8241a01d7
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: geevensingh

CHANGELOG.md

@@ -36,7 +36,12 @@ body. Keep section headings exact and write notes in Markdown.
   position on every mouse-move frame instead of waiting for the
   editor's scroll + layout pass to round-trip — keeps the indicator
   truly stuck to the cursor even on large files where the editor
-  scroll can't quite keep up at fast drag speeds.
+  scroll can't quite keep up at fast drag speeds. Clicking empty
+  bar space (anywhere that isn't a hunk marker or the viewport
+  indicator) recenters the indicator on the cursor and starts a
+  drag in the same gesture — minimap-style "click anywhere to jump
+  there", and a press-and-drag from empty space scrubs without
+  needing a second click.
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DiffViewer/Rendering/HunkOverviewBar.cs

@@ -25,7 +25,10 @@ namespace DiffViewer.Rendering;
 ///   <item>Hover tooltip shows the hunk's old- and new-side line ranges so
 ///   the user can scan without clicking. Clicking a marker (or the ribbon
 ///   between two markers) jumps the editors to that hunk via
-///   <see cref="DiffPaneViewModel.JumpToHunk"/>.</item>
+///   <see cref="DiffPaneViewModel.JumpToHunk"/>. Clicking empty bar space
+///   recenters the viewport band on the cursor (minimap-style "jump to
+///   here") and immediately enters a sticky-thumb drag so the same press
+///   can continue scrubbing without releasing the button.</item>
 ///   <item>Cluster markers, popups, and keyboard focus are listed in the
 ///   plan but deferred to a follow-up — the hit-test still falls through
 ///   to the nearest marker when two rects overlap.</item>
@@ -359,8 +362,30 @@ protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
             _vm.Viewport, leftTotal, rightTotal, ActualWidth, ActualHeight, ColumnWidth);
         bool inBand = band is not null && HunkOverviewBarGeometry.IsInsideBand(band, p);
 
-        // Empty press — let it bubble.
-        if (idx < 0 && !inBand) return;
+        // Empty press: minimap-style "click anywhere to jump there".
+        // The band's center snaps to the cursor immediately, then any
+        // continued motion drags from there — empty-space press and
+        // empty-space-press-then-drag are the same gesture, sharing
+        // the existing sticky-thumb path. Skipping PendingClick and
+        // going straight to Dragging means the first scroll fires on
+        // MouseDown (so a quick click lands the band on the cursor
+        // even without any mouse movement). If there's no band yet
+        // (no file loaded / pre-layout) we bubble instead — nothing
+        // to center on.
+        if (idx < 0 && !inBand)
+        {
+            if (band is null) return;
+            _bandDragOffsetFromTop = HunkOverviewBarGeometry.GetBandHeight(band) / 2.0;
+            _interactionStartPoint = p;
+            _pendingHunkIndex = -1;
+            _bandWasUnderClick = false;
+            _interaction = BarInteractionState.Dragging;
+            ToolTip = null;
+            CaptureMouse();
+            EmitDragScroll(p);
+            e.Handled = true;
+            return;
+        }
 
         // Defer both the JumpToHunk commit and the drag start to
         // MouseMove/MouseUp so a click that lands on a hunk marker