Variable frametime during user input

## Bevy version

0.7

## Operating system & version

Win11

## What you did

Use the tracing features to observe frame timings.

## What you expected to happen

Frametime should be consistent with an empty app.

## What actually happened

Extreme variability in the un-instrumented time between runs of the system graph while moving the mouse.

## Additional information

Using immediate mode (vsync off).
I started moving my mouse in the second half of the capture:


A closer look at the time in question:

Reproduce the issue in a Bevy 0.7 empty app on Windows 11, using the tracing features to capture frame timings. Compare an idle run with a run while moving the mouse, keeping immediate mode enabled and vsync disabled. Done means the unexplained variability in un-instrumented system-graph time is identified and frametimes are consistent or the remaining limitation is documented.

Written by the indexing model from the issue text.