invoke idle priority timers only when actually idle

The 'Idle' timers are misnamed. They are zero-timeout times, i.e.
they are invoked immediately after returning to the main loop.
But that does not necessarily mean they are invoked when idle,
there may be e.g. user input pending in the system event queue.
In fact, LO events are processed before system events, which means
that 'Idle' timers are normally processed before user input.

Besides being confused, this also leads to poor performance in some
cases, such as when using mouse wheel to zoom in a large document.
This results in several mouse wheel events, each of which will
result in adjusting the zoom and that causing a repaint. Repaints
are internally handled using a TaskPriority::REPAINT 'Idle',
and so what happens is zoom->repaint->zoom->repaint->zoom->repaint
instead of the more efficient zoom->zoom->zoom->repaint.

This change (besides trying to clarify the confusion in the docs)
delays invoking tasks with priorities TaskPriority::HIGH_IDLE
and lower if there is user input or repaint events in the OS queue.
That means that tasks using idle priorities actually will be invoked
only when idle (barring background threads etc.).

I'm reasonably certain this is a safe change, there's no guarantee
when exactly tasks will be invoked (e.g. other tasks with a higher
priority go first) and explicitly specifying such a priority means
asking for it.

I already implemented this once in 06d731428ef6cf93c7333e8228b,
and it was also again done in 87199d3829257420429057336283, but
apparently these have been removed. There was d348035a60361a1b9ba9e
'Drop special idle handling' with the reasoning that 'Idles are just
instant timers'. Which strictly technically speaking is true due to
'Idle' being a misnomer, but the point is that some idles should be
actual idles and that's why they need to be handled specially.

Change-Id: I36c2b02a80ae7e1476b731f878d9b28aa87975f4
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/110538
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/vcl/task.hxx b/include/vcl/task.hxx
index d8adae7eff0b..e95aa5bc460a 100644
--- a/include/vcl/task.hxx
+++ b/include/vcl/task.hxx
@@ -28,6 +28,7 @@ enum class TaskPriority
 {
     HIGHEST,       ///< These events should run very fast!
     DEFAULT,       ///< Default priority used, e.g. the default timer priority
+    // Input from the OS event queue is processed before HIGH_IDLE tasks.
     HIGH_IDLE,     ///< Important idle events to be run before processing drawing events
     RESIZE,        ///< Resize runs before repaint, so we won't paint twice
     REPAINT,       ///< All repaint events should go in here