Do not block visualization manager updates when opening the display panel dialog

[ivanpauno]

Fixes #793.

[clalancette]

In theory I'm on board with this, but you might want to have a look at ros-visualization/rviz#631, ros-visualization/rviz#1606, ros-visualization/rviz#1639, and ros-visualization/rviz#1641 where similar things have been tossed around in the original RViz for years. Your suggested solution is different, so it may solve the problems, but we should do some testing of the other problems reported.

[ivanpauno]

According to #601, the idea of stopping the world when openning a dialog was intentional.
I don't see how that solves the reported problem:

This fixes the problem that rviz becomes effectively unusable when the update/render cycle takes too long (e.g. while showing high-frequency point cloud data).

was the idea to open a dialog when rviz gets unusable so you can remove the display causing the issue?

If that was the intention, it looks like a really weird idea.
The solution applied to noetic looks better, though I would like to test ros-visualization/rviz#1641.

[clalancette]

was the idea to open a dialog when rviz gets unusable so you can remove the display causing the issue?

Good question; I don't know :).

If that was the intention, it looks like a really weird idea.

But yeah, I agree, it seems weird.

[ivanpauno]

I've (partially) reverted ros-visualization/rviz#631.
I've tried to reproduce the issue reported in ros-visualization/rviz#1641 and I couldn't.

I think that deleting this stopUpdate()/startUpdate() calls is fine.
If this reintroduces a problem, blocking updates while a dialog is open is probably not the right solution and we will have to figure out something else (or if that's the right solution, maybe we want to be spinning a ros executor somewhere else).

[ivanpauno]

@jacobperron friendly ping

[jacobperron]

lgtm

[ivanpauno]

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