Slow DOM Change with v-show - vue.js

Question

I've been troubleshooting an issue where, inside of a nested v-for loop, a simple v-show directive is taking up toward 4 seconds to actually apply the display: none; attribute in the DOM.
The basic layout is this. I've changed the actual naming to be all about cars; because why not!
I do apologize for the wall of code, but I felt it was relevant to include a basic jist of the component structure.
<div class="vehicle-history" v-if="fetchComplete && !fetchError">
<h3 class="header-bold">History</h3>
<div class="history-container" v-for="car in vehicles.cars" :key="car.id">
<div class="vehicle-header">
<div class="toyota" v-if="car.model == 'toyota'">
<span class="model-id">#{{ car.type }}_#{{ car.id }}</span> - #{{ car.title }}
</div>
<div class="audi" v-if="car.model == 'audi'">
<span class="model-id">#{{ car.type }}_#{{ car.id }}</span> - #{{ car.title }}
</div>
<div class="benz" v-if="car.model == 'benz'">
<span class="model-id">#{{ car.type }}_#{{ car.id }}</span> - #{{ car.title }}
</div>
<div class="rolls" v-if="car.model == 'rolls'">
<span class="model-id">#{{ car.type }}_#{{ car.id }}</span> - #{{ car.title }}
</div><br />
<small v-for="color in car.colors">#{{ color.name }} </small>
</div>
<div class="vehicle-body">
<div class="description">#{{ car.description }}</div>
<div class="review-container" v-if="car.reviews.length > 5">
<div class="review" v-for="n in (0, 4)" :key="car.reviews[n].id">
<strong>#{{ car.reviews[n].rating }}</strong> - #{{ car.reviews[n].content }}<br />
<small>#{{ car.reviews[n].created_at.date }} #{{ car.reviews[n].created_at.timezone }}</small>
<span v-show="n == 4 && !car.displayFull"><br />
<button type="button"
class="btn btn-link"
data-toggle="collapse"
v-on:click="car.displayFull = true"
v-bind:data-target="'#expanded-reviews-' + car.id">More details
</button>
</span>
</div>
<div class="collapse" v-bind:id="'expanded-reviews-' + car.id">
<div class="review" v-for="n in (4, car.reviews.length-1)" :key="car.reviews[n].id">
<strong>#{{ car.reviews[n].rating }}</strong> - #{{ car.reviews[n].content }}<br />
<small>#{{ car.reviews[n].created_at.date }} #{{ car.reviews[n].created_at.timezone }}</small>
<span v-if="n == car.reviews.length-1"><br />
<button type="button"
class="btn btn-link"
data-toggle="collapse"
v-on:click="car.displayFull = false"
v-bind:data-target="'#expanded-reviews-' + car.id">Less details
</button>
</span>
</div>
</div>
</div>
</div>
</div>
</div>
What I've tried
Adding the :key to the relevant loops
Moving the v-show attribute from the button (where it was originally at) to the parent span.
Wrapping it in <keep-alive> (which didn't work at all, it broke visibility as a whole)
Swapping v-show for v-if and vice-versa
The expected output
What I'm trying to accomplish is to make sure the button for "More details" hides away when the accordion gets expanded (that is, when the "More details" button is initially clicked) and re-appears if the "Less details" button is clicked.
The actual output
Whilst it does the above, there's a delay of between 2-5 seconds from clicking the button, to it getting the display: none; attribute applied in the DOM. The change to the Vue variable however appears instant.
Do you have any clue why this may be, and if there's a good workaround for this? Thank you, as always!
UPDATE
Revised code example below based on a number of suggestions. This is a flat out copy of existing code, so no changes to naming this time around.
<div class="incident-history text-center col-xs-12 padding-bottom" v-if="fetchComplete && !fetchError">
<h3 class="heading3 black-text">Incident History</h3>
<div class="incident-container" v-for="incident in responseData.incident.resolved" :key="incident.id">
<div class="incident-header">
<div :class="'title-' + incident.classification">
<span class="incident-id">#{{ incident.type }}_#{{ incident.id }}</span> - #{{ incident.title }}
</div><br />
<small v-for="monitor in incident.monitors">#{{ monitor.name }} </small>
</div>
<div class="incident-body">
<div class="description">#{{ incident.content }}</div>
<div class="update" v-for="(comment, index) in incident.comments.slice(0,5)">
<strong>#{{ comment.type }}</strong> - #{{ comment.content }}<br />
<small>#{{ comment.created_at.date }} #{{ comment.created_at.timezone }}</small>
</div>
<div class="collapse"
v-bind:id="'expanded-update-' + incident.id"
v-if="incident.comments.length > 4">
<div class="update" v-for="(comment, index) in incident.comments.slice(5)">
<strong>#{{ comment.type }}</strong> - #{{ comment.content }}<br />
<small>#{{ comment.created_at.date }} #{{ comment.created_at.timezone }}</small>
</div>
</div>
<button type="button"
class="btn btn-link"
data-toggle="collapse"
v-show="incident.comments.length > 4 && !incident.displayFull"
v-on:click="incident.displayFull = !incident.displayFull"
v-bind:data-target="'#expanded-update-' + incident.id">Show more
</button>
<button type="button"
class="btn btn-link"
data-toggle="collapse"
v-show="incident.comments.length > 4 && incident.displayFull"
v-on:click="incident.displayFull = !incident.displayFull"
v-bind:data-target="'#expanded-update-' + incident.id">Show less
</button>
</div>
</div>
</div>
I will note that the above code block is run inside of yet another for-loop. The basic topology is this (lengths may vary)
responseData: Object
incident: Object
active: Array[4]
0: Object
1: Object
Property: SomeValue
Property: SomeValue
Property: SomeValue
Property: SomeValue
Property: SomeValue
Comments: Array[3]
0: Object
1: Object
Property: SomeValue
Property: SomeValue
2: Object
2: Object
3: Object
Alternative Solution
I've come up with a working, and likely better alternative for the changing of "Read more" to "Read less" and vice versa.
button.full-details[aria-expanded="true"]:after {
content: 'Show less';
}
button.full-details[aria-expanded="false"]:after {
content: 'Show more';
}
This works fine, and just needs adding a few lines of CSS as well as a class to the button (or targeting the Bootstrap button class, but that's not great for many reasons).
That said, I'm going to leave this one open for a little bit to hopefully gain an understanding into why the issue arose with Vue in the first place.

Answer 1

It is surprising to hear 2-5 secs lag in display, but then there's quite a bit going on in that template. Hard to tell if suggested changes will impact the page visuals, without a test system.
The button(s) seems most problematic, creates one per review but only needs one per car, has to evaluate v-show or v-if for all of them.
Have one button outside the reviews loop,
<button class="btn btn-link" type="button"
data-toggle="collapse" :data-target="'#expanded-reviews-' + car.id"
#click="car.displayFull = !car.displayFull">
{{ buttonPrompt(car.displayFull) }}
</button>
I'd also consider creating a car child component, even a review grandchild component, as reasoning about the layout becomes difficult when it's that busy.
The final thing is so obvious, I'm thinking it must be an artifact of the domain renaming.
This
<div class="toyota" v-if="car.model == 'toyota'">
<span class="model-id">#{{ car.type }}_#{{ car.id }}</span> - #{{ car.title }}
</div>
<div class="audi" v-if="car.model == 'audi'">
<span class="model-id">#{{ car.type }}_#{{ car.id }}</span> - #{{ car.title }}
</div>
<div class="benz" v-if="car.model == 'benz'">
<span class="model-id">#{{ car.type }}_#{{ car.id }}</span> - #{{ car.title }}
</div>
<div class="rolls" v-if="car.model == 'rolls'">
<span class="model-id">#{{ car.type }}_#{{ car.id }}</span> - #{{ car.title }}
</div>
could become this
<div :class="car.model">
<span class="model-id">{{ car.type }}_{{ car.id }}</span> - {{ car.title }}
</div>
Also, what is this? #{{?
Edit after comment - on the question of how many buttons are created
It sounds like you are expecting only one or two buttons to be rendered? Here's a test.
From a spec for Cars.vue created with your template,
Test data has 6 reviews,
<script>
export default {
data () {
return {
fetchComplete: true,
fetchError: false,
cars: [
{
model: 'toyota',
type: 'hatch',
id: 1,
title: 'corolla',
description: 'grey corolla',
colors: [{name: 'grey'}, {name: 'grey'}, {name: 'grey'}],
reviews: [
{rating: 2, content: 'its grey', created_at: {date: ''}},
{rating: 2, content: 'its grey', created_at: {date: ''}},
{rating: 2, content: 'its grey', created_at: {date: ''}},
{rating: 2, content: 'its grey', created_at: {date: ''}},
{rating: 2, content: 'its grey', created_at: {date: ''}},
{rating: 2, content: 'its grey', created_at: {date: ''}},
]
}
]
}
},
The test,
it('count the buttons', () => {
const buttons = wrapper.element.querySelectorAll('button');
console.log(buttons)
})
Results,
LOG LOG: Object{
0: <button type="button" data-toggle="collapse" data-target="#expanded-reviews-1" class="btn btn-link">More details</button>,
1: <button type="button" data-toggle="collapse" data-target="#expanded-reviews-1" class="btn btn-link">More details</button>,
2: <button type="button" data-toggle="collapse" data-target="#expanded-reviews-1" class="btn btn-link">More details</button>,
3: <button type="button" data-toggle="collapse" data-target="#expanded-reviews-1" class="btn btn-link">More details</button>,
4: <button type="button" data-toggle="collapse" data-target="#expanded-reviews-1" class="btn btn-link">Less details</button>,
length: 5}
Then, there's the question of whether the change detection cycle is smart enough not to re-evaluate button creation each cycle. I'll see if we can test that too.
Placement of v-if
It just occurred to me, this
<button class="btn btn-link" ... >
<span v-if="n === car.reviews.length-1">Less details</span>
</button>
might be better as this
<button class="btn btn-link" ... v-if="n === car.reviews.length-1">
Less details
</button>
Missing first review
The v-for="n in (0, 4)" loop seems to miss out the first review. If I set the ratings to be sequential numbers, the first is not displayed.
<div class="review" v-for="n in (0, 4)">
<strong>#{{ car.reviews[n].rating }}</strong> - #{{ car.reviews[n].content }}
Try
<div class="review" v-for="(review,n) of car.reviews.slice(0,5)" :key="review.id">
<strong>#{{ review.rating }}</strong> - #{{ review.content }}
...
<div class="review" v-for="(review,n) of car.reviews.slice(5)">
(keeping n for button v-if expression).