Certain element won't fade in when I scroll past a certain portion of a section

Question

I have a little problem with my code.

As you can see, my example has a three sections. More exactly landing-page , section and about . For the last two I used CSS grid to split them each in two subsections.

I'm trying to achieve a transition on scroll using jquery . Like, when I scroll past a 1/3 of section , my left element starts to fade from opacity:0 . Respectively this should happen with my right element from my about section, but with my code this happens as soon as I scroll past a 1/3 of section .

I tried changing the class of my right.hidden to right.hide and it stopped fading completely.

Bellow I've attached my code and here is a link to my CodePen

Jade

.landing-page
.section
  .left.hidden
  .right
.about
  .left
  .right.hidden 

Sass

.landing-page
  height: 100vh
  width: 100vw
  background: gray
.section
  height: 100vh
  width: 100vw
  display: grid
  grid-template-columns: repeat(2, 1fr)
  grid-template-areas: 'left right' 'left right'
  .left
    grid-area: left
    background: orangered
    transition: 2000ms
  .left.hidden
    opacity: 0
  .right
    grid-area: right
    background: lightblue
.about
  height: 100vh
  width: 100vw
  display: grid
  grid-template-columns: repeat(2, 1fr)
  grid-template-areas: 'left right' 'left right'
  .left
    grid-area: left
    background: lightgreen
  .right
    grid-area: right
    background: orangered
    transition: 2000ms
  .right.hidden
    opacity: 0

Jquery

$(document).ready(function() {
  var sectionLeftEl = $('.left'),
      sectionRightEl = $('.right'),
      sectionLeftElOffset = sectionLeftEl.offset().top / 3,
      sectionRightElOffset = sectionRightEl.offset().top / 3,
      documentEl = $(document);

  documentEl.on('scroll', function() {
    if (documentEl.scrollTop() > sectionLeftElOffset && sectionLeftEl.hasClass('hidden')) sectionLeftEl.removeClass('hidden');
    if (documentEl.scrollTop() > sectionRightElOffset && sectionRightEl.hasClass('hidden') sectionRightEl.removeClass('hidden');

  });
});

Answer 1

If I understand you correctly, the calculation is wrong (additional of the selectors issue which @Temani Afif mention).

The section offset (right or left) should be

sectionTopOffset - 2 / 3 * sectionHeight

Just imagine (or test it) that when you start to scroll, you can see the top of .section . So you want that whenever you exceed the .section in the 1/3 it's like to say 2 / 3 * .section.height()

So:

 $(document).ready(function() { var sectionLeftEl = $('.section .left'), sectionRightEl = $('.about .right'), sectionLeftElOffset = sectionLeftEl.offset().top - (2 * sectionLeftEl.height() / 3), sectionRightElOffset = sectionRightEl.offset().top - (2 * sectionRightEl.height() / 3), documentEl = $(document); documentEl.on('scroll', function() { if (documentEl.scrollTop() > sectionLeftElOffset && sectionLeftEl.hasClass('hidden')) sectionLeftEl.removeClass('hidden'); if (documentEl.scrollTop() > sectionRightElOffset && sectionRightEl.hasClass('hidden')) sectionRightEl.removeClass('hidden'); }); }); 

 .landing-page { height: 100vh; width: 100vw; background: gray; } .section { height: 100vh; width: 100vw; display: grid; grid-template-columns: repeat(2, 1fr); grid-template-areas: "left right" "left right"; } .section .left { grid-area: left; background: orangered; transition: 2000ms; } .section .left.hidden { opacity: 0; } .section .right { grid-area: right; background: lightblue; } .about { height: 100vh; width: 100vw; display: grid; grid-template-columns: repeat(2, 1fr); grid-template-areas: "left right" "left right"; } .about .left { grid-area: left; background: lightgreen; } .about .right { grid-area: right; background: orangered; transition: 2000ms; } .about .right.hidden { opacity: 0; } 

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <div class="landing-page"></div> <div class="section"> <div class="left hidden"></div> <div class="right"></div> </div> <div class="about"> <div class="left"></div> <div class="right hidden"></div> </div> 

https://codepen.io/moshfeu/pen/jxmYyp?editors=0010