A navigator drag keeps its drop marker until it lands, leaves the panel or is cancelled

[LilianBarbe]

Illustration of the navigator panel, not a screenshot.

The bug

While dragging a row in the navigator, the insertion line flickers and often disappears. NavigatorBody clears the drop target on every dragleave, and dragleave also fires when the pointer moves between two children of the panel: from one row to the next, or onto the marker itself. The marker also takes up layout space and catches the pointer, which makes it worse.

The fix

• The drop target is cleared only when the pointer really leaves the panel (relatedTarget outside the body), hovers blank space, or the drag ends (drop or Esc).
• .drop-indicator is absolutely positioned inside its gap with pointer-events: none, so it no longer shifts the gap or becomes a drag target.

Two UI choices, easy to drop if you'd rather not

Each one is a single line in StructureTree.tsx:

1. No native drag ghost. setDragImage gets a transparent 1×1 image, so no copy of the row sits under the pointer and hides the rows around the drop point. Only the drop line shows where the node will land. This depends on the fix above: with a flickering line, you would lose track of the drop point.
2. Starting a drag selects the row. Without the ghost, the highlighted row shows what is being moved.

Tests

• test/navigator.js gets 8 new checks: the marker survives moving onto itself and onto a row, and it clears on leaving the panel, on blank space and on dragend. Without the fix, 5 of them fail. With it, all 49 pass.
• test:navigatoropencode passes and tsc --noEmit reports nothing.

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