comctl32/trackbar: Fix reverting to default value for TBM_SETPAGESIZE with lParam = -1.

dlls/comctl32/tests/trackbar.c

@@ -588,12 +588,26 @@ static void test_page_size(HWND hWndTrackbar){
 
     /* test TBM_GETPAGESIZE */
     r = SendMessage(hWndTrackbar, TBM_GETPAGESIZE, 0,0);
-    todo_wine{
-        expect(20, r);
-    }
+    expect(20, r);
 
     ok_sequence(sequences, TRACKBAR_SEQ_INDEX, page_size_test_seq, "page size test sequence", FALSE);
     ok_sequence(sequences, PARENT_SEQ_INDEX, parent_empty_test_seq, "parent page size test sequence", FALSE);
+
+    /* check for zero page size */
+    r = SendMessage(hWndTrackbar, TBM_SETPAGESIZE, 0, 0);
+    expect(20, r);
+    r = SendMessage(hWndTrackbar, TBM_GETPAGESIZE, 0, 0);
+    expect(0, r);
+    /* revert to default */
+    r = SendMessage(hWndTrackbar, TBM_SETPAGESIZE, 0, -1);
+    expect(0, r);
+    r = SendMessage(hWndTrackbar, TBM_GETPAGESIZE, 0, 0);
+    expect(20, r);
+    /* < -1 */
+    r = SendMessage(hWndTrackbar, TBM_SETPAGESIZE, 0, -2);
+    expect(20, r);
+    r = SendMessage(hWndTrackbar, TBM_GETPAGESIZE, 0, 0);
+    expect(-2, r);
 }
 
 static void test_position(HWND hWndTrackbar){

dlls/comctl32/trackbar.c

@@ -81,6 +81,8 @@ typedef struct
 
 #define TOOLTIP_OFFSET		2     /* distance from ctrl edge to tooltip */
 
+#define TB_DEFAULTPAGESIZE	20
+
 /* Used by TRACKBAR_Refresh to find out which parts of the control
    need to be recalculated */
 
@@ -1128,7 +1130,10 @@ TRACKBAR_SetPageSize (TRACKBAR_INFO *infoPtr, LONG lPageSize)
 {
     LONG lTemp = infoPtr->lPageSize;
 
-    infoPtr->lPageSize = lPageSize;
+    if (lPageSize != -1)
+        infoPtr->lPageSize = lPageSize;
+    else
+        infoPtr->lPageSize = TB_DEFAULTPAGESIZE;
 
     return lTemp;
 }
@@ -1405,7 +1410,7 @@ TRACKBAR_Create (HWND hwnd, const CREATESTRUCTW *lpcs)
     infoPtr->lRangeMin = 0;
     infoPtr->lRangeMax = 100;
     infoPtr->lLineSize = 1;
-    infoPtr->lPageSize = 20;
+    infoPtr->lPageSize = TB_DEFAULTPAGESIZE;
     infoPtr->lSelMin   = 0;
     infoPtr->lSelMax   = 0;
     infoPtr->lPos      = 0;