[LyX/master] Fix algorithm that computes horizontal scroll offset

[Jean-Marc Lasgouttes]

commit 1f0d210ab56057f35960a3b86f5fa1e03ef8ecd0
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Tue Oct 27 16:11:01 2015 +0100

    Fix algorithm that computes horizontal scroll offset
    
    Rewrite the logic completely:
    * fix cases where the offset was reset unnecessarily
    * fix cases where the row was scrolled too much: as soon as a side of the 
row is completely visible, there is no need to scroll more.
    * fix cases where offset would never reset

diff --git a/src/BufferView.cpp b/src/BufferView.cpp
index 658fa0e..7287ad8 100644
--- a/src/BufferView.cpp
+++ b/src/BufferView.cpp
@@ -3042,18 +3042,27 @@ void BufferView::checkCursorScrollOffset(PainterInfo & 
pi)
 
        // Horizontal scroll offset of the cursor row in pixels
        int offset = d->horiz_scroll_offset_;
-       int const MARGIN = 2 * 
theFontMetrics(d->cursor_.real_current_font).em();
-       //lyxerr << "cur_x=" << cur_x << ", offset=" << offset << ", margin=" 
<< MARGIN << endl;
-       if (cur_x < offset + MARGIN) {
-               // scroll right
-               offset = cur_x - MARGIN;
-       } else if (cur_x > offset + workWidth() - MARGIN) {
-               // scroll left
-               offset = cur_x - workWidth() + MARGIN;
+       int const MARGIN = 2 * theFontMetrics(d->cursor_.real_current_font).em()
+                          + row.right_margin;
+       if (row.width() <= workWidth() - row.right_margin) {
+               // Row is narrower than the work area, no offset needed.
+               offset = 0;
+       } else {
+               if (cur_x - offset < MARGIN) {
+                       // cursor would be too far right
+                       offset = cur_x - MARGIN;
+               } else if (cur_x - offset > workWidth() - MARGIN) {
+                       // cursor would be too far left
+                       offset = cur_x - workWidth() + MARGIN;
+               }
+               // Correct the offset to make sure that we do not scroll too 
much
+               if (offset < 0)
+                       offset = 0;
+               if (row.width() - offset < workWidth() - row.right_margin)
+                       offset = row.width() - workWidth() + row.right_margin;
        }
 
-       if (offset < row.left_margin || row.width() <= workWidth())
-               offset = 0;
+       //lyxerr << "cur_x=" << cur_x << ", offset=" << offset << ", row.wid=" 
<< row.width() << ", margin=" << MARGIN << endl;
 
        if (offset != d->horiz_scroll_offset_)
                LYXERR(Debug::PAINTING, "Horiz. scroll offset changed from "