Fix: limit scrolling to drawing + perimeter.

Before this patch, the scroll limits were arbitrary, which
made the user have to use zoom to recenter the scroll perimeter,
in order to navigate the whole document.

This patch limits scrolling to the drawing instead. The exact perimeters
are now so that the drawing is always in the viewport, with an extra
empty zone of 75% for potentially extending the drawing.

This issue got mentioned in LibreCAD#479

librecad/src/ui/qg_graphicview.cpp

@@ -51,7 +51,7 @@
 #include "rs_modification.h"
 #include "rs_debug.h"
 
-#define QG_SCROLLMARGIN 400
+//#define QG_SCROLLMARGIN 400
 
 #ifdef Q_OS_WIN32
 #define CURSOR_SIZE 16
@@ -685,7 +685,6 @@ void QG_GraphicView::keyReleaseEvent(QKeyEvent* e)
     eventHandler->keyReleaseEvent(e);
 }
 
-
 /**
  * Called whenever the graphic view has changed.
  * Adjusts the scrollbar ranges / steps.
@@ -727,20 +726,21 @@ void QG_GraphicView::adjustOffsetControls()
             max = RS_Vector(100,100);
         }
 
+        auto factor = getFactor();
 
-        int minVal = (int)(-ox-getWidth()*0.5
-                           - QG_SCROLLMARGIN - getBorderLeft());
-        int maxVal = (int)(-ox+getWidth()*0.5
-                           + QG_SCROLLMARGIN + getBorderRight());
+        int minVal = (int)(-getWidth()*0.75
+                           + std::min(min.x, 0.)*factor.x);
+        int maxVal = (int)(-getWidth()*0.25
+                           + std::max(max.x, 0.)*factor.x);
 
         if (minVal<=maxVal) {
             hScrollBar->setRange(minVal, maxVal);
         }
 
-        minVal = (int)(oy-getHeight()*0.5
-                       - QG_SCROLLMARGIN - getBorderTop());
-        maxVal = (int)(oy+getHeight()*0.5
-                       +QG_SCROLLMARGIN + getBorderBottom());
+        minVal = (int)(+getHeight()*0.25
+                       - std::max(max.y, 0.)*factor.y);
+        maxVal = (int)(+getHeight()*0.75
+                       - std::min(min.y, 0.)*factor.y);
 
         if (minVal<=maxVal) {
             vScrollBar->setRange(minVal, maxVal);