user32: edit control scrolling follows selection

[Henry Kroll III]

Fixes Bug 22274 (patch and test program attached to the bug.)

* Scroll horizontally by thumb size, not single characters.
* Don't scroll vertically when selection grows horizontally.
* Stop selection from growing while mouse is stationary.

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diff --git a/dlls/user32/edit.c b/dlls/user32/edit.c
index 9898bc4..ebe0ae3 100644
--- a/dlls/user32/edit.c
+++ b/dlls/user32/edit.c
@@ -3796,15 +3796,19 @@ static LRESULT EDIT_WM_SysKeyDown(EDITSTATE *es, INT key, DWORD key_data)
  */
 static void EDIT_WM_Timer(EDITSTATE *es)
 {
-	if (es->region_posx < 0) {
-		EDIT_MoveBackward(es, TRUE);
-	} else if (es->region_posx > 0) {
-		EDIT_MoveForward(es, TRUE);
-	}
-/*
- *	FIXME: gotta do some vertical scrolling here, like
- *		EDIT_EM_LineScroll(hwnd, 0, 1);
- */
+        INT fw = es->format_rect.right - es->format_rect.left;
+        INT max_x_offset = es->text_width - fw;
+        INT page = fw / HSCROLL_FRACTION / es->char_width * es->char_width;
+        
+        /* scroll to selection */
+        INT dx = es->region_posx * page;
+        INT dy = es->region_posy;
+    
+        /* sanity check */
+        if (dx + es->x_offset > max_x_offset)
+                dx = max_x_offset - es->x_offset;
+
+        if (dx || dy) EDIT_EM_LineScroll_internal(es, dx, dy);
 }
 
 /*********************************************************************