DeskTop: Fix repaint of icons dragged on desktop

Regressed in 1bdfff3. When optimizing repaints after dragging icons
within the some window (or desktop), it failed to specify that erasing
required clipping when for desktop icons moves.

res/notes/testplan.md

@@ -103,6 +103,7 @@
 * Open a window. Position two icons so one overlaps another. Select only one icon. Drag it to a new location. Verify that the the both icons repaint correctly.
 
 * Position a volume icon in the middle of the DeskTop. Incrementally move a window so that it obscures all 8 positions around it (top, top right, right, etc). Ensure the icon repaints fully, and no part of the window is over-drawn.
+* Position a window partially overlapping desktop icons. Select overlapped desktop icons. Drag icons a few pixels to the right. Verify that window is not over-drawn.
 
 * Launch DeskTop, File > Quit, run BASIC.SYSTEM. Ensure /RAM exists.
 

toolkits/icontk.s

@@ -873,6 +873,7 @@ includes_trash:
 
         ;; Drag within same window (or desktop)
 same_window:
+        ldx     #$80            ; clip (if desktop)
         lda     findwindow_params::which_area
         .assert MGTK::Area::desktop = 0, error, "enum mismatch"
         beq     move_ok
@@ -888,10 +889,12 @@ same_window:
 
         ;; --------------------------------------------------
 
+        ldx     #0              ; don't clip (not desktop; unnecessary)
 move_ok:
-
+        stx     ::drag_highlighted_lambda_clip_flag
         INVOKE_WITH_LAMBDA IterateHighlightedIcons
-        ldx     #0              ; don't clip
+        ::drag_highlighted_lambda_clip_flag := *+1
+        ldx     #SELF_MODIFIED_BYTE
         clc                     ; don't redraw highlighted
         jmp     EraseIconCommon ; A = icon id, X = clip flag, C = redraw flag
         END_OF_LAMBDA