Software renderer renders bottom line when a GUI is out of screen

[ericoporto]

Describe the bug
When using software mode, if a GUI is Visible, but positioned with Y in excess of the screen height, the bottom of the screen will produce a black line. You can move the mouse over the line to view a small part of the line.

AGS Version
3.6.0.38

Game
Create a new game using the Sierra Template, and in the Project Explorer, select the gStatusline GUI, and in properties, set it's Y position to 201 or bigger. In default setup, chose the Software Renderer. Alternatively, download the project below.
SoftwareRendererBug.zip

To Reproduce
Steps to reproduce the behavior:

1. Move the mouse to the bottom of the screen, and notice a black line there
2. Where the mouse goes over, the black line is removed

Expected behavior
There should be no line at the bottom

Screenshots
Note the bottom line.

Desktop:

• OS: Windows
• Version 10

Additional context
Originally reported here: https://www.adventuregamestudio.co.uk/forums/index.php?topic=60323.msg636651847#msg636651847

[ericoporto]

I noticed in invalidate_rect_on_surf, it clamps the position of the dirty rects, and I also noticed we put sprite on screen using a check but we always mark a region with dirty, with no check for boundaries. Commenting this invalidate_sprite at line 2380 seems to fix this specific case.

I think the problem is here:

ags/Engine/ac/draw_software.cpp

Lines 352 to 353 in cca997f

	if (!IsRectInsideRect(rects.Viewport, r))
	invalidate_rect_on_surf(x1, y1, x2, y2, BlackRects);

Basically, we get to a condition where the rect is not in the viewport (it's outside of the screen), but it's not over the black border either. But it then it gets on the screen because of the clamping.

The below appears to fix, but I am not sure if it causes new problems - could not find any so far.

diff --git a/Engine/ac/draw_software.cpp b/Engine/ac/draw_software.cpp
index e1469aae1..947900859 100644
--- a/Engine/ac/draw_software.cpp
+++ b/Engine/ac/draw_software.cpp
@@ -256,6 +256,13 @@ void invalidate_all_camera_rects(int view_index)
     RoomCamRects[view_index].NumDirtyRegions = WHOLESCREENDIRTY;
 }

+void swap_pos(int &a, int &b)
+{
+    int tmp = a;
+    a = b;
+    b = tmp;
+}
+
 void invalidate_rect_on_surf(int x1, int y1, int x2, int y2, DirtyRects &rects)
 {
     if (rects.DirtyRows.size() == 0)
@@ -266,9 +273,15 @@ void invalidate_rect_on_surf(int x1, int y1, int x2, int y2, DirtyRects &rects)
         return;
     }

+    if(x1 > x2) swap_pos(x1, x2);
+    if(y1 > y2) swap_pos(y1, y2);
+
     int a;

     const Size &surfsz = rects.SurfaceSize;
+
+    if(x1 >= surfsz.Width || y1 >= surfsz.Height || x2 < 0 || y2 < 0) return;
+
     if (x1 >= surfsz.Width) x1 = surfsz.Width - 1;
     if (y1 >= surfsz.Height) y1 = surfsz.Height - 1;
     if (x2 >= surfsz.Width) x2 = surfsz.Width - 1;