selection scrolling should only depend on y value (ghostty-org#8686)

Fixes ghostty-org#8683

The selection scrolling logic should only depend on the y value of the
cursor position, not the x value. This presents unwanted scroll
behaviors, such as reversing the scroll direction which was just a side
effect of attempting to scroll tick to begin with.

Author: mitchellh

src/Surface.zig

@@ -4012,7 +4012,7 @@ pub fn cursorPosCallback(
 
     // Stop selection scrolling when inside the viewport within a 1px buffer
     // for fullscreen windows, but only when selection scrolling is active.
-    if (pos.x >= 1 and pos.y >= 1 and self.selection_scroll_active) {
+    if (pos.y >= 1 and self.selection_scroll_active) {
         self.io.queueMessage(
             .{ .selection_scroll = false },
             .locked,