From 965ab4293877ecda02793782fab504019ccb28b5 Mon Sep 17 00:00:00 2001
From: Tomasz Wlostowski <tomasz.wlostowski@cern.ch>
Date: Wed, 15 Dec 2021 01:46:03 +0100
Subject: [PATCH] router: when dragging segments, don't snap to other segments
 on the same layer. Fixes drag flickering for short segments.

Fixes: https://gitlab.com/kicad/code/kicad/-/issues/9720
---
 pcbnew/router/pns_router.cpp    | 3 ++-
 pcbnew/router/pns_router.h      | 1 +
 pcbnew/router/pns_tool_base.cpp | 8 ++++++++
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/pcbnew/router/pns_router.cpp b/pcbnew/router/pns_router.cpp
index 8980626a55..12afdca5ac 100644
--- a/pcbnew/router/pns_router.cpp
+++ b/pcbnew/router/pns_router.cpp
@@ -161,6 +161,7 @@ bool ROUTER::StartDragging( const VECTOR2I& aP, ITEM_SET aStartItems, int aDragM
     {
         m_dragger = std::make_unique<COMPONENT_DRAGGER>( this );
         m_forceMarkObstaclesMode = true;
+        m_state = DRAG_COMPONENT;
     }
     else
     {
@@ -170,6 +171,7 @@ bool ROUTER::StartDragging( const VECTOR2I& aP, ITEM_SET aStartItems, int aDragM
             m_forceMarkObstaclesMode = false;
 
         m_dragger = std::make_unique<DRAGGER>( this );
+        m_state = DRAG_SEGMENT;
     }
 
     m_dragger->SetMode( aDragMode );
@@ -187,7 +189,6 @@ bool ROUTER::StartDragging( const VECTOR2I& aP, ITEM_SET aStartItems, int aDragM
 
     if( m_dragger->Start( aP, aStartItems ) )
     {
-        m_state = DRAG_SEGMENT;
         return true;
     }
     else
diff --git a/pcbnew/router/pns_router.h b/pcbnew/router/pns_router.h
index 5053a472d9..5b47cdad97 100644
--- a/pcbnew/router/pns_router.h
+++ b/pcbnew/router/pns_router.h
@@ -119,6 +119,7 @@ public:
     {
         IDLE,
         DRAG_SEGMENT,
+        DRAG_COMPONENT,
         ROUTE_TRACK
     };
 
diff --git a/pcbnew/router/pns_tool_base.cpp b/pcbnew/router/pns_tool_base.cpp
index 984e92829e..c0c5a74177 100644
--- a/pcbnew/router/pns_tool_base.cpp
+++ b/pcbnew/router/pns_tool_base.cpp
@@ -243,6 +243,14 @@ bool TOOL_BASE::checkSnap( ITEM *aItem )
     // Sync PNS engine settings with the general PCB editor options.
     auto& pnss = m_router->Settings();
 
+    // If we're dragging a track segment, don't try to snap to items on the same copper layer.
+    // This way we avoid 'flickery' behaviour for short segments when the snap algo is trying to
+    // snap to the corners of the segments next to the one being dragged.
+    if( m_startItem && aItem && m_router->GetState() == ROUTER::DRAG_SEGMENT
+        && aItem->Layer() == m_startItem->Layer() && aItem->OfKind( ITEM::SEGMENT_T )
+        && m_startItem->OfKind( ITEM::SEGMENT_T ) )
+        return false;
+
     pnss.SetSnapToPads(
             frame()->GetMagneticItemsSettings()->pads == MAGNETIC_OPTIONS::CAPTURE_CURSOR_IN_TRACK_TOOL ||
             frame()->GetMagneticItemsSettings()->pads == MAGNETIC_OPTIONS::CAPTURE_ALWAYS );