Fix bug #8837: LyX hangs on selection

The code that checks whether the cursor was at the end of a row in
Cursor::upDowninText was not able to set boundary correctly. This
causes a hang in because the cursor got stuck on a line and there is an
infinite loop BufferView::dispatch when trying to go down.

The fix is to avoid using the watered-down TextMetrics::x2pos wrapper
around getColumnNearX and use the real thing instead.

Eventually, the last user of x2pos (InsetTabular) should be fixed and
the method should go away.

src/Cursor.cpp

@@ -2024,17 +2024,13 @@ bool Cursor::upDownInText(bool up, bool & updateNeeded)
 				next_row = 0;
 			}
 		}
-		top().pos() = min(tm.x2pos(pit(), next_row, xo), top().lastpos());
 
-		int const xpos = tm.x2pos(pit(), next_row, xo);
-		bool const at_end_row = xpos == tm.x2pos(pit(), next_row, tm.width());
-		bool const at_beg_row = xpos == tm.x2pos(pit(), next_row, 0);
-
-		if (at_end_row && at_beg_row)
-			// make sure the cursor ends up on this row
-			boundary(false);
-		else
-			boundary(at_end_row);
+		Row const & real_next_row = tm.parMetrics(pit()).rows()[next_row];
+		bool bound = false;
+		pos_type const col = tm.getColumnNearX(pit(), real_next_row, 
+						       xo, bound);
+		top().pos() = real_next_row.pos() + col;
+		boundary(bound);
 
 		updateNeeded |= bv().checkDepm(*this, old);
 	}

status.20x

@@ -83,12 +83,15 @@ What's new
 
 - Fix crash when navigating to next change (bug 8684).
 
-- Fix crash when optional argument inside a math macro was deleted (bug 8329).
+- Fix crash when optional argument inside a math macro was deleted
+  (bug 8329).
+
+- Fix hang when selecting text (bug 8837).
 
 - Fix math-ams-matrix function that could corrupt documents if not used properly
   (part of bug 8359).
 
-- Fix problem that led to assertion in some cases when space was at beginning of 
+- Fix problem that led to assertion in some cases when space was at beginning of
   line (bugs 8838 and 8947).
 
 - Correctly compare documents with different author sets (bug 8769).