Fix text selection when history size limit is exceeded in client console

Selection is currently wandering when part of history is trimmed. Size
difference before and after trimming must be taken into account to
prevent this.

Client/core/CConsole.cpp

@@ -543,10 +543,43 @@ void CConsole::FlushPendingAdd ( void )
         m_strPendingAdd = "";
 
         // Trim new buffer
-        if ( strBuffer.length () > CONSOLE_SIZE )
+        uint uiBufferLength = strBuffer.length ();
+
+        if ( uiBufferLength > CONSOLE_SIZE )
         {
             strBuffer = strBuffer.Right ( CONSOLE_SIZE );
             strBuffer = strBuffer.SplitRight ( "\n" );
+
+            // Fix text selection coords after trimming
+            if ( uiSelectionLength > 0 )
+            {
+                uint uiBufferLengthDiff = uiBufferLength - strBuffer.length ();
+
+                // Beware of underflows, all cases must be properly handled
+                if ( uiSelectionEnd < uiBufferLengthDiff )
+                {
+                    // Whole selection would be out of the screen now, so technically
+                    // it does not exist anymore
+                    uiSelectionStart = 0;
+                    uiSelectionEnd = 0;
+                    uiSelectionLength = 0;
+                }
+                else if ( uiSelectionStart < uiBufferLengthDiff )
+                {
+                    // Start of selection would be out of the screen now,
+                    // so it must be shortened
+                    uiSelectionLength -= uiBufferLengthDiff - uiSelectionStart;
+                    uiSelectionStart = 0;
+                    uiSelectionEnd -= uiBufferLengthDiff;
+                }
+                else // Both start and end of selection are greater than length difference
+                {
+                    // Whole selection would still be visible on the screen,
+                    // just a simple movement is needed
+                    uiSelectionStart -= uiBufferLengthDiff;
+                    uiSelectionEnd -= uiBufferLengthDiff;
+                }
+            }
         }
 
         // Set new buffer
@@ -556,7 +589,7 @@ void CConsole::FlushPendingAdd ( void )
         if ( fScroll < fMaxScroll )
             m_pHistory->SetVerticalScrollPosition ( fScroll );
 
-        // Keep text selection if any
+        // Restore text selection if any
         if ( uiSelectionLength > 0 )
             m_pHistory->SetSelection ( uiSelectionStart, uiSelectionEnd );
     }