[LyX/master] Stabilize mouse selection in mathed

[Richard Heck]

commit 302228625b46788d4301e9bbdc2883b4e581f508
Author: Juergen Spitzmueller <sp...@lyx.org>
Date:   Fri Apr 4 15:40:05 2014 +0200

    Stabilize mouse selection in mathed
    
    Fixes: #9074

diff --git a/src/mathed/InsetMathNest.cpp b/src/mathed/InsetMathNest.cpp
index 479a682..83e49c4 100644
--- a/src/mathed/InsetMathNest.cpp
+++ b/src/mathed/InsetMathNest.cpp
@@ -1567,16 +1567,29 @@ void InsetMathNest::lfunMousePress(Cursor & cur, 
FuncRequest & cmd)
 void InsetMathNest::lfunMouseMotion(Cursor & cur, FuncRequest & cmd)
 {
        // only select with button 1
-       if (cmd.button() == mouse_button::button1) {
-               Cursor & bvcur = cur.bv().cursor();
-               if (bvcur.realAnchor().hasPart(cur)) {
-                       //lyxerr << "## lfunMouseMotion: cursor: " << cur << 
endl;
-                       bvcur.setCursor(cur);
-                       bvcur.setSelection(true);
-                       //lyxerr << "MOTION " << bvcur << endl;
-               } else
-                       cur.undispatched();
+       if (cmd.button() != mouse_button::button1)
+               return;
+
+       Cursor & bvcur = cur.bv().cursor();
+
+       // ignore motions deeper nested than the real anchor
+       if (!bvcur.realAnchor().hasPart(cur)) {
+               cur.undispatched();
+               return;
        }
+
+       CursorSlice old = bvcur.top();
+
+       // We continue with our existing selection or start a new one, so don't
+       // reset the anchor.
+       bvcur.setCursor(cur);
+       // Did we actually move?
+       if (cur.top() == old)
+               // We didn't move one iota, so no need to change selection 
status
+               // or update the screen.
+               cur.screenUpdateFlags(Update::SinglePar | Update::FitCursor);
+       else
+               bvcur.setSelection();
 }