Recalculate layout when legend item size changes

[m-kuhn]

This triggers an update() of the legend viewport when the size of the legend item changes.

The solution is only partially satisfying. This should be done automatically by the model / view themselves. And it seems to work correctly for WMS. I couldn't find out why.

This patch works without any side effects. Happy to dump it in favor of an alternative approach.

Fixes #38881

[nyalldawson]

Could you test with a project involving 1000s of layers and confirm there's no regressions in project handling speed here? I've previously seen that legend update code like this can cause temporary qgis hangs on large projects (that's an ongoing issue, but I don't want it to get any worse then it currently is!)

[m-kuhn]

What is the requirement exactly? Can I duplicate a vector layer 1000 times and add an AMS layer and test with that?

[nyalldawson]

That'd be a good test. Also throw a vector layer with rule based renderer or categorised renderer and 10000 categories in there.

[m-kuhn]

I'm confident this doesn't introduce any noticeable performance impact. It triggers a (deferred) repaint like any user interface interaction does (clicking somewhere, hitting a random key on the keyboard, scrolling, ...) and only does this under a specific condition.

I'll try to create meaningful results. It fear it won't be easy.

Talking about... Debugging the legend code I've seen code paths triggered repeatedly without any good reason, so if you say you've observed bad performance ... I'm not surprised.

[m-kuhn]

Confirmed with 10000 categories legend, 34 seconds to add the layer with and without the patch.

[m-kuhn]

Followup issue #38890

[nyalldawson]

Thanks, proceed at will!