How do you add UI inside cells in a google spreadsheet using app script?

I'd like to add buttons to specific cells in Google docs spreadsheet. The apps script UI documentation talks about how to add a new panel, but it's not clear how UI in that panel could be attached to specific rows or cells.

Is it possible to add UI to particular cells, or are we limited to adding new panels?

The apps UI only works for panels.

The best you can do is to draw a button yourself and put that into your spreadsheet. Than you can add a macro to it.

Go into "Insert > Drawing...", Draw a button and add it to the spreadsheet. Than click it and click "assign Macro...", then insert the name of the function you wish to execute there. The function must be defined in a script in the spreadsheet.

Alternatively you can also draw the button somewhere else and insert it as an image.

More info: https://developers.google.com/apps-script/guides/menus

Status 2018:

There seems to be no way to place buttons (drawings, images) within cells in a way that would allow them to be linked to Apps Script functions.

---

This being said, there are some things that you can indeed do:

You can...

You can place images within cells using IMAGE(URL), but they cannot be linked to Apps Script functions.

You can place images within cells and link them to URLs using:
=HYPERLINK("http://example.com"; IMAGE("http://example.com/myimage.png"; 1))

You can create drawings as described in the answer of @Eduardo and they can be linked to Apps Script functions, but they will be stand-alone items that float freely "above" the spreadsheet and cannot be positioned in cells. They cannot be copied from cell to cell and they do not have a row or col position that the script function could read.

Buttons can be added to frozen rows as images. Assigning a function within the attached script to the button makes it possible to run the function. The comment which says you can not is of course a very old comment, possibly things have changed now.

Use checkboxes(say, in F1) instead of buttons/Images. This is a intermediate to a full ui inside cells. Then hook your function, that is supposed to run on button click to onEdit() trigger function.

Sample script:

function onEdit(e){
  const rg = e.range;
  if(rg.getA1Notation() === "F1" && rg.isChecked() && rg.getSheet().getName() === "Sheet1"){
    callFunctionAttachedToImage();
    rg.uncheck();
  }
}

References:

• Class Range
• Event Objects