LBTK: Fix corruption when clicking in blank space below items

A flag was not preserved. Regressed in 4dc3386.

res/notes/testplan.md

@@ -1364,7 +1364,7 @@ Watch out for:
  * Caret being placed in the wrong place by a click.
  * Mispaint when cleared with Control+X (or Clear key on IIgs)
 
-# List Boxes Controls
+# List Box Controls
 
 This covers:
 * File Pickers
@@ -1395,7 +1395,6 @@ Repeat for each list box:
     * Selects the last item.
 * Hold down the mouse button on the scrollbar's up arrow or down arrow; verify that the scrolling continues as long as the button is held down.
 * Click on an item. Verify it is selected. Click on white space below the items (if possible). Verify that selection is cleared.
-* Hold the down arrow to scroll to the bottom of the list. Hold the up arrow to scroll to the top. Verify the correct items redraw.
 
 For the Sounds DA:
 * Click on an item. Verify it is selected, and plays the sound. Click on the same selected item. Verify it plays the sound again.

toolkits/lbtk.s

@@ -507,8 +507,8 @@ new_selection   .byte
         pha                     ; A = new selection
         lda     lbr_copy + LBTK::ListBoxRecord::selected_index
         jsr     _HighlightIndex
-        pla                     ; A = new selection
         ldy     #LBTK::ListBoxRecord::selected_index
+        pla                     ; A = new selection
         sta     (lbr_ptr),y
         sta     lbr_copy + LBTK::ListBoxRecord::selected_index ; keep copy in sync
         bmi     :+
@@ -710,6 +710,8 @@ update:
         lda     lbr_copy + LBTK::ListBoxRecord::num_items
         beq     finish
 
+        index := tmp_point + .sizeof(MGTK::Point)
+
         lda     lbr_copy + LBTK::ListBoxRecord::num_rows
         sta     rows
         ldy     #MGTK::Winfo::vthumbpos
@@ -723,8 +725,7 @@ update:
 loop:
         MGTK_CALL MGTK::MoveTo, tmp_point
 
-        index := *+1
-        lda     #SELF_MODIFIED_BYTE
+        lda     index
         ldxy    #tmp_point
         jsr     DrawEntryProc