From 12eb71ee4e0987e9a57d163db7fd342b4bd174bb Mon Sep 17 00:00:00 2001
From: wp_xxyyzz <wp_xxyyzz@8e941d3f-bd1b-0410-a28a-d453659cc2b4>
Date: Wed, 12 Mar 2025 09:46:35 +0000
Subject: [PATCH] LazMapViewer: Fix loss of coordinate accuracy when dragging
 points at small zoom factor. Issue #39114.

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@9675 8e941d3f-bd1b-0410-a28a-d453659cc2b4
---
 components/lazmapviewer/source/mvdragobj.pas  | 27 ++++++++++++++++---
 .../lazmapviewer/source/mvmapviewer.pas       | 11 ++++----
 2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/components/lazmapviewer/source/mvdragobj.pas b/components/lazmapviewer/source/mvdragobj.pas
index 3539e4adc..d3679c430 100644
--- a/components/lazmapviewer/source/mvdragobj.pas
+++ b/components/lazmapviewer/source/mvdragobj.pas
@@ -38,13 +38,16 @@ Type
     FEndX,FEndY : integer;
     FOnDrag: TDragEvent;
     FOnEndDrag: TDragEvent;
+    function GetEndPt: TPoint;
+    function GetOfsPt: TPoint;
+    function GetStartPt: TPoint;
 
     procedure SetDest(X,Y : Integer);
     procedure SetLnkObj(AValue: TObject);
     procedure SetOnDrag(AValue: TDragEvent);
     procedure SetOnEndDrag(AValue: TDragEvent);
 
-    Procedure DostartDrag(X,Y: Integer);
+    Procedure DoStartDrag(X,Y: Integer);
     Procedure DoDrag(X,Y: integer);
     Procedure DoEndDrag(X,Y: integer);
     Function HasMoved(X,Y: integer) : Boolean;
@@ -58,12 +61,15 @@ Type
       property OnDrag: TDragEvent read FOnDrag write SetOnDrag;
       property OnEndDrag: TDragEvent read FOnEndDrag write SetOnEndDrag;
 
+      property OfsPt: TPoint read GetOfsPt;
       property OfsX: integer read FOfsX;
       property OfsY: integer read FOfsY;
+      property StartPt: TPoint read GetStartPt;
       property StartX: integer read FStartX;
       property StartY: integer read FStartY;
       property MouseX: Integer read FMouseX;
       property MouseY: integer read FMouseY;
+      property EndPt: TPoint read GetEndPt;
       property EndX: integer read FEndX;
       property EndY: integer read FEndY;
       Property LnkObj: TObject Read FLnkObj write SetLnkObj;
@@ -130,6 +136,21 @@ begin
   FInDrag := False;
 end;
 
+function TDragObj.GetEndPt: TPoint;
+begin
+  Result := Point(FEndX, FEndY);
+end;
+
+function TDragObj.GetOfsPt: TPoint;
+begin
+  Result := Point(FOfsX, FOfsY);
+end;
+
+function TDragObj.GetStartPt: TPoint;
+begin
+  Result := Point(FStartX, FStartY);
+end;
+
 function TDragObj.HasMoved(X, Y: integer): Boolean;
 begin
   Result := (X <> FStartX) or (Y <> FStartY);
@@ -139,10 +160,10 @@ procedure TDragObj.AbortDrag;
 begin
   if FInDrag then
   Begin
-    DoDrag(FstartX,FStartY);
+    DoDrag(FStartX, FStartY);
     FInDrag := False;
     FMouseDown := False;
-    FDragSrc :=nil;
+    FDragSrc := nil;
     FStartSrc := nil;
     FreeAndNil(FLnkObj);
   end;
diff --git a/components/lazmapviewer/source/mvmapviewer.pas b/components/lazmapviewer/source/mvmapviewer.pas
index c7062c00c..eea234531 100644
--- a/components/lazmapviewer/source/mvmapviewer.pas
+++ b/components/lazmapviewer/source/mvmapviewer.pas
@@ -4781,7 +4781,7 @@ end;
 procedure TMapEditMark.DoDrag(Sender: TDragObj);
 var
   I: Integer = 0;
-  RPt: TRealPoint;
+  RptStart, RPtEnd: TRealPoint;
   MapPoint: TMapPoint;
 begin
   if not FDragStarted then
@@ -4804,10 +4804,11 @@ begin
   for MapPoint in FSelection.Points do
   begin
     MarkDirty;
-    Rpt := FMapView.ScreenToLatLon(FMapView.LatLonToScreen(FOrigins[I]) +
-      Point(Sender.OfsX, Sender.OfsY));
-    MapPoint.Longitude := RPt.Lon;
-    MapPoint.Latitude := RPt.Lat;
+    // Apply positions of drag start and end points from pixel space to real world space
+    RPtStart := FMapView.ScreenToLatLon(Sender.StartPt);
+    RPtEnd := FMapView.ScreenToLatLon(Sender.EndPt);
+    MapPoint.Longitude := FOrigins[I].Lon + (RPtEnd.Lon - RPtStart.Lon);
+    MapPoint.Latitude := FOrigins[I].Lat + (RPtEnd.Lat - RPtStart.Lat);
     Inc(I);
   end;
   //FMapView.Invalidate; // No need to