From c4fa30ac7ddf529e489fd3d59f50b19c26f59f7c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jonas=20Dre=C3=9Fler?= <verdre@v0yd.nl>
Date: Sun, 8 Dec 2019 12:02:51 +0100
Subject: [PATCH] backends/native: Fix relative motion calculation if monitor
 isn't found

The method `relative_motion_across_outputs` is used to adjust the
distance/delta of a mouse movement across multiple monitors to take the
different scale factors of those monitors into account. This works by
getting the adjacent monitors that the movement-line/vector intersects
with and adjusting the final position (or end point of the
movement-line) by multiplying the parts of the line spanning across
different monitors with the scale factors of those monitors.

In the end of this calculation, we always want to set the new end
coordinates of the relative motion to the new end coordinates of the
adjusted movement-line. We currently only do that if all adjacent
monitors the line is crossing actually exist, because only then we end
up inside the "We reached the dest logical monitor" else-block and set
`x` and `y` to the correct values. Fix that and make sure the returned
values are also correct in case an adjacent monitor doesn't exist by
adding separate `target_x` and `target_y` variables which we update during
each pass of the while loop so we're always prepared for the while loop
exiting before the destination monitor was found.

Thanks to Axel Kittenberger for reporting the initial bug and tracking
the issue down to `relative_motion_across_outputs`.

Fixes https://gitlab.gnome.org/GNOME/mutter/issues/774
---
 src/backends/native/meta-backend-native.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/src/backends/native/meta-backend-native.c b/src/backends/native/meta-backend-native.c
index 5e4a8dd97..469cd0cc1 100644
--- a/src/backends/native/meta-backend-native.c
+++ b/src/backends/native/meta-backend-native.c
@@ -227,6 +227,7 @@ relative_motion_across_outputs (MetaMonitorManager *monitor_manager,
 {
   MetaLogicalMonitor *cur = current;
   float x = cur_x, y = cur_y;
+  float target_x = cur_x, target_y = cur_y;
   float dx = *dx_inout, dy = *dy_inout;
   MetaDisplayDirection direction = -1;
 
@@ -256,6 +257,9 @@ relative_motion_across_outputs (MetaMonitorManager *monitor_manager,
               { cur->rect.x + cur->rect.width, cur->rect.y + cur->rect.height }
       };
 
+      target_x = motion.b.x;
+      target_y = motion.b.y;
+
       if (direction != META_DISPLAY_RIGHT &&
           meta_line2_intersects_with (&motion, &left, &intersection))
         direction = META_DISPLAY_LEFT;
@@ -269,12 +273,8 @@ relative_motion_across_outputs (MetaMonitorManager *monitor_manager,
                meta_line2_intersects_with (&motion, &bottom, &intersection))
         direction = META_DISPLAY_DOWN;
       else
-        {
-          /* We reached the dest logical monitor */
-          x = motion.b.x;
-          y = motion.b.y;
-          break;
-        }
+        /* We reached the dest logical monitor */
+        break;
 
       x = intersection.x;
       y = intersection.y;
@@ -285,8 +285,8 @@ relative_motion_across_outputs (MetaMonitorManager *monitor_manager,
                                                                cur, direction);
     }
 
-  *dx_inout = x - cur_x;
-  *dy_inout = y - cur_y;
+  *dx_inout = target_x - cur_x;
+  *dy_inout = target_y - cur_y;
 }
 
 static void