Understanding grid negative values

Question

I\'m trying to make a grid that has a full span row at the bottom.

For full span columns I can use grid-column: 1/-1.

For single span columns I

Answer 1

when using the same value inside grid-column/grid-row you will fall into this rule:

If the placement for a grid item contains two lines, and the start line is further end-ward than the end line, swap the two lines. If the start line is equal to the end line, remove the end line.^ref

So saying grid-column:-1/-1 means grid-column:-1 which is grid-column:-1/auto

auto

The property contributes nothing to the grid item’s placement, indicating auto-placement or a default span of one. (See § 8 Placing Grid Items, above.)

So basiclly you said to your element to start at the last line and span one column which will create an implicit new column:

A basic example to illustrate:

.box {
  display:grid;
  grid-template-columns:20px 20px 20px;
  grid-auto-columns:100px;
  grid-gap:5px;
}

span {
  grid-column:-1/-1;
  height:40px;
  background:red;
}

You can see that the span is having 100px which means it create a new column inside the implicit grid and is not inside the explicit one defined by 20px

When using -2/-1 it's clear that you will consider the before the last and the last line and the element will be placed in the last explicit column:

.box {
  display:grid;
  grid-template-columns:20px 20px 20px;
  grid-auto-columns:100px;
  grid-gap:5px;
}

span {
  grid-column:-2/-1;
  height:40px;
  background:red;
}

Same logic apply when using positive value but you won't notice a strange behavior since you will most likely span an explicit column thinking it's correct to specify, for example, grid-column:1/1