vo_opengl: win32: try to enable DwmFlush by default

Enable it by default, but not unconditionally. Add an "auto" mode, which
disable DwmFlush if the compositor is (probably) inactive. Let's see how
this goes.

Since I accidentally enabled DwmFlush always by default (more or less)
in a previous commit touching this code, this is probably mostly just
cargo-culting, and it's uncertain whether it does anything.

Note that I still got bad vsync behavior when fullscreening mpv, and
making another window visible on the same screen. This happens even if
forcing DWM.

1 files changed, 6 insertions, 2 deletions

diff --git a/DOCS/man/vo.rst b/DOCS/man/vo.rst
index 2cebd153ff..b2a6cd7045 100644
--- a/DOCS/man/vo.rst
+++ b/DOCS/man/vo.rst
@@ -676,11 +676,15 @@ Available video output drivers are:
         the number of frames the GPU queues ahead (which also has an influence
         on vsync).
 
-    ``dwmflush=<no|windowed|yes>``
-        Calls ``DwmFlush`` after swapping buffers on Windows (default: no).
+    ``dwmflush=<no|windowed|yes|auto>``
+        Calls ``DwmFlush`` after swapping buffers on Windows (default: auto).
         It also sets ``SwapInterval(0)`` to ignore the OpenGL timing. Values
         are: no (disabled), windowed (only in windowed mode), yes (also in
         full screen).
+
+        The value ``auto`` will try to determine whether the compositor is
+        active, and calls ``DwmFlush`` only if it seems to be.
+
         This may help getting more consistent frame intervals, especially with
         high-fps clips - which might also reduce dropped frames. Typically a
         value of ``windowed`` should be enough since full screen may bypass the