Optimize relation combobox panel search against large lists

[nirvn]

Attempt at improving things enough for #7007 to be considered fixed.

@FelixHinckel , can you give this a try? The first strike is still slow-ish but I suspect on lists that are not 100,000 items should now be usable.

[nirvn]

A bit of background on this: prior to QField 3.7, we would filter and sort through repeated data provider requests. We moved to a full list we filter on the Qt side via a filter proxy model.

You can see in the filed issue that we also have a lag pre 3.7, but that's not blocking the UI.

[qfield-fairy]

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[FelixHinckel]

I tried on the same project, and a modified project where I filtered the list to ≈ 20 000 entries (the actual data I have to use at minima in my projects).

I see some improvements on the smaller list, but the lag is still observable.

From what I see, the more you write the more the lag diminishes. I imagine that with a tighter filter, the app has less data to handle. But that is hard for the user as he cannot see the beginning of what he's typing (and possibly typos).

I tried (with the relation value widget) to restrict the amount of results shown, but this doesn't apply to QField.

(For a bit of context, I moved from relation values to reference values for a "speed of search" improvement. If the relation values were to become faster, taht would be good to me too.)

[nirvn]

@FelixHinckel , I never noticed this function -- what are the uses for limiting a relation reference list?

[FelixHinckel]

None, I just thought that showing only the 5 first results could decrease the lag.

[FelixHinckel]

Do you think this issue will be handled ? I have to say that it is quite important for me, as it will drive the development of the projet I am leading.

[FelixHinckel]

I just checked the latest commit, it works well!
Will it be kept like this in the next releases ?

[nirvn]

@FelixHinckel , it'll be part of the next release yes, circa end of March 2026. We could backport this but I'm a little bit reluctant at this stage.

[FelixHinckel]

No worries, that is just to inform the users! March is good!

[FelixHinckel]

Hi @nirvn !
This beta works perfectly fine for the filters, I'm gonna use it until the next official release.
However, I've found some bugs here that aren't present in the 4.0.5. Do I report them ? And if so, where ?

[nirvn]

@FelixHinckel please do report them on GitHub. Just mention you're on master. Thanks!