Edited the comment explaining why the logic in DisplayText needs

to be changed for macOS.

1 files changed, 21 insertions, 11 deletions

diff --git a/generic/tkTextDisp.c b/generic/tkTextDisp.c
index d2b6147..24184e3 100644
--- a/generic/tkTextDisp.c
+++ b/generic/tkTextDisp.c
@@ -4443,18 +4443,28 @@ DisplayText(
 #ifdef MAC_OSX_TK
 	    } else if (dlPtr->chunkPtr != NULL) {
 		/*
-		 * On the Mac we need to redisplay all embedded windows which
-		 * were moved in order to make sure that the clipping region
-		 * for the Text widget is maintained correctly.  If an embedded
-		 * window is moved but its bounding rectangle is not added back
-		 * to the clipping region of the Text widget then drawing into
-		 * the old bounding rectangle will not be allowed, causing an
-		 * image of the embedded window to remain on the screen in the
-		 * old position. In other words, a "ghost window" will appear.
+		 * On macOS we need to redisplay all embedded windows which
+		 * were moved by the call to TkScrollWindows above.  This is
+		 * not necessary on Unix or Windows because XScrollWindow will
+		 * have included the bounding rectangles of all of these
+		 * windows in the damage region.  The macosx implementation of
+		 * TkScrollWindow does not do this.  It simply generates a
+		 * damage region which is the scroll source rectangle minus
+		 * the scroll destination rectangle.  This is because there is
+		 * no efficient process available for iterating through the
+		 * subwindows which meet the scrolled area.  (On Unix this is
+		 * handled by GraphicsExpose events generated by XCopyArea and
+		 * on Windows by ScrollWindowEx.  On macOS the low level
+		 * scrolling is accomplished by calling [view scrollRect:by:].
+		 * This method does not provide any damage information and, in
+		 * any case, could not be aware of Tk windows which were not
+		 * based on NSView objects.
 		 *
-		 * Perhaps all platforms should do this, rather than allow the
-		 * embedded windows to have incorrect values for Tk_X and Tk_Y.
-		 * But apparently this is not known to cause problems.
+		 * On the other hand, this loop is already iterating through
+		 * all embedded windows which could possibly have been moved
+		 * by the scrolling.  So it is as efficient to redisplay them
+		 * here as it would have been if they had been redisplayed by
+		 * the call to TextInvalidateRegion above.
 		 */
 #else
 	    } else if (dlPtr->chunkPtr != NULL && ((dlPtr->y < 0)