Two way binding triggers reactivity twice

[ruslan-khomiak]

Describe the bug

When the application is first launched, binding of non-primitive types is triggered twice

Reproduction

https://svelte.dev/repl/4a49ea25eca94484974c2f4c0f3d294b?version=3.42.3

Logs

No response

System Info

System:
    OS: macOS 10.15.6
    CPU: (8) x64 Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
    Memory: 2.11 GB / 16.00 GB
    Shell: 3.2.57 - /bin/bash
  Binaries:
    Node: 15.9.0 - /usr/local/bin/node
    npm: 7.5.3 - /usr/local/bin/npm
  Browsers:
    Chrome: 92.0.4515.159
    Edge: 92.0.902.78
    Firefox: 90.0.2
    Safari: 13.1.2
  npmPackages:
    svelte: ^3.40.0 => 3.40.1 
    webpack: ^5.46.0 => 5.46.0

Severity

annoyance

[dummdidumm]

The underlying reason might be the same as for #6016

[Prinzhorn]

We have recently collected a couple of duplicates here #6590

[dummdidumm]

Thanks for pointing that out. Closing as duplicate of #5689

[RaiVaibhav]

Hey @dummdidumm summary of what I found, this issue is quite similar #6590 and the reason is because of the two-way binding, so for case like bind:clientwidth reactive will retrigger but for case where parent state bind with child state like in #5689 but it values is not getting changed then, svelte, not able to figure out whether to re-trigger or not, it's simply re-trigger every time.
besides this, #5689 is different from above and the reason is because of the conditional check not_equal($$.ctx[i], $$.ctx[i] = value) and when value is {} it makes the component dirty comparatively to when value is of primitive data type and there is one more point I didn't understand if (ready) make_dirty(component, i); so in the #5689 there a state change inside the reactive and svelte wait for its first reactive update till then it's ready, but what if there is state change inside the first reactive update.

I have opened the PR for fixing the beforeUpdate part.