[gtk: 1/2] Fix frameclock going backwards

[Matthias Clasen <matthiasc src gnome org>]

commit a36e2bc764ba493a07c982995e1f76eac53a9383
Author: Alexander Larsson <alexl redhat com>
Date:   Mon Jun 1 10:53:24 2020 +0200

    Fix frameclock going backwards
    
    When we run the frameclock RUN_FLUSH_IDLE idle before the paint,
    then gdk_frame_clock_flush_idle() sets
    ```
      priv->phase = GDK_FRAME_CLOCK_PHASE_BEFORE_PAINT
    ```
    at the end if there is a paint comming.
    
    But, before doing the paint cycle it may handle other X events, and
    during that time the phase is set to BEFORE_PAINT. This means that the
    current check on whether we're inside a paint is wrong:
    
    ```
      if (priv->phase != GDK_FRAME_CLOCK_PHASE_NONE &&
         priv->phase != GDK_FRAME_CLOCK_PHASE_FLUSH_EVENTS)
        return priv->smoothed_frame_time_base;
    ```
    
    This caused us to sometimes use this smoothed_frame_time_base even
    though we previously reported a later value during PHASE_NONE, thus
    being non-monotonic.
    
    We can't just additionally check for the BEGIN_PAINT phase though,
    becasue if we are in the paint loop actually doing that phase we
    should use the time base. Instead we check for `!(BEFORE_PAINT &&
    in_paint_idle)`.

 gdk/gdkframeclockidle.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/gdk/gdkframeclockidle.c b/gdk/gdkframeclockidle.c
index bbb4e2d26e..ef32ad518a 100644
--- a/gdk/gdkframeclockidle.c
+++ b/gdk/gdkframeclockidle.c
@@ -236,7 +236,8 @@ gdk_frame_clock_idle_get_frame_time (GdkFrameClock *clock)
 
   /* can't change frame time during a paint */
   if (priv->phase != GDK_FRAME_CLOCK_PHASE_NONE &&
-      priv->phase != GDK_FRAME_CLOCK_PHASE_FLUSH_EVENTS)
+      priv->phase != GDK_FRAME_CLOCK_PHASE_FLUSH_EVENTS &&
+      (priv->phase != GDK_FRAME_CLOCK_PHASE_BEFORE_PAINT || priv->in_paint_idle))
     return priv->smoothed_frame_time_base;
 
   /* Outside a paint, pick something smoothed close to now */