EdgeHandle : Tooltip track edge, not cursor

Changes.md

@@ -15,7 +15,8 @@ Improvements
 - Spreadsheet :
   - Popups for string cells and row names are now sized to fit their column.
   - Added "Triple" and "Quadruple" width options to the spreadsheet row name popup menu.
-- LightTool : Changed spot light tool tip location so that it follows the cone during drag.
+- LightTool :
+  - Changed spot light and quad light edge tool tip locations so that they follow the cone and edge during drag.
 
 1.3.4.0 (relative to 1.3.3.0)
 =======

src/GafferSceneUI/LightTool.cpp

@@ -2058,6 +2058,7 @@ class EdgeHandle : public LightToolHandle
 			m_edgeScale( 1.f ),
 			m_oppositeScale( 1.f ),
 			m_orientation(),
+			m_tooltipT( 0 ),
 			m_oppositeAdditionalScale( 1.f )
 		{
 		}
@@ -2245,8 +2246,14 @@ class EdgeHandle : public LightToolHandle
 			edgeSegment.p1 += offset;
 			edgeSegment *= m_orientation;
 
-			V3f eventClosest;
-			setTooltipPosition( edgeSegment.closestPoints( LineSegment3f( eventLine.p0, eventLine.p1 ), eventClosest ) );
+			if( !m_drag )
+			{
+				V3f eventClosest;
+				const V3f closestPoint = edgeSegment.closestPoints( LineSegment3f( eventLine.p0, eventLine.p1 ), eventClosest );
+				m_tooltipT = ( closestPoint - edgeSegment.p0 ).length() / edgeSegment.length();
+			}
+
+			setTooltipPosition( edgeSegment( m_tooltipT ) );
 		}
 
 	private :
@@ -2318,6 +2325,7 @@ class EdgeHandle : public LightToolHandle
 		float m_oppositeScale;
 		M44f m_orientation;
 		float m_oppositeAdditionalScale;
+		float m_tooltipT;  // Parameter `t` along the edge set at the start of the drag
 		std::optional<LinearDrag> m_drag;
 };