MacViews: Correct behavior of move and select commands when selection direction changes.

ui/gfx/text_constants.h

Index: ui/gfx/text_constants.h
diff --git a/ui/gfx/text_constants.h b/ui/gfx/text_constants.h
index 93450ea56070d6957379e15733ac24ab14c2d131..65b7adb6e8c6b0b33e4008753a542a0d20928e68 100644
--- a/ui/gfx/text_constants.h
+++ b/ui/gfx/text_constants.h
@@ -15,6 +15,27 @@ enum BreakType {
   LINE_BREAK,          // Stop cursor movement on line ends as shown on screen.
 };
 
+// Specifies how a move and select command behaves if the selection direction
+// is reversed. For example consider the state "ab|cd|e", i.e. cd is selected.
+// Assume the selection direction is from left to right. If we move to the
+// beginning of the line while modifying the selection, the resultant state is:
+// "|ab|cde" for SELECTION_START_NEW, selection direction from right to left.
+// "|abcd|e" for SELECTION_EXTEND, selection direction from right to left.
+// "ab|cde" for SELECTION_CARET.
+enum SelectionReversedBehavior {
+  // Existing selection is deselcted and a new selection is started in the
+  // opposite direction. Default.
+  SELECTION_START_NEW,
+
+  // The existing selection is extended in the opposite direction.

[tapted, 2016/05/20 06:47:36]

Might be good to give an example of a particular case this is for. E.g. "This is
the behavior for textfields on Mac when doing <blah>". Same for SELECTION_CARET

[karandeepb, 2016/05/24 07:47:15]

Done.

+  SELECTION_EXTEND,
+
+  // The existing selection reduces to a caret.
+  SELECTION_CARET,
+
+  SELECTION_DEFAULT = SELECTION_START_NEW,
+};
+
 // Specifies the word wrapping behavior when a word would exceed the available
 // display width. All words that are too wide will be put on a new line, and
 // then: