Why does mousing over one button also highlight adjacent buttons?

Question

I'm using some buttons, and want to arrange them in a row. That is, some will be directly next to each other. I don't want whitespace between them. This is easy enough to do:

However, when I do this, mousing over either of them highlights both. I would expect only the moused-over button to be highlighted.

Using this code:

(progn (insert ?\n)
       (insert-text-button "one")
       (insert " ")
       (insert-text-button "two")
       (insert " ")
       (insert-text-button "three")
       (insert-text-button "four"))

Here is a screenshot of what happens when I mouse over a button with whitespace after it. Notice that only "one" is highlighted.

And here is a screenshot of what happens when I mouse over a button with a button immediately after it. It highlights the button "three", which is expected, but also the button "four", which is not.

How can I make Emacs only highlight "three", the button being moused over?

Answer 1

Buttons are based on text properties. Each character in a buffer has its own properties. Emacs doesn't record start and end positions for properties: properties are not intervals. When it needs to know where a property starts and ends, it looks for the previous or next change in property. Likewise, buttons don't have a recorded start and end: instead all the characters that make up the button have the same button-related properties.

Buttons are highlighted when the mouse cursor hovers over them because they have the mouse-face property set to highlight. Emacs highlights the text extent around the character under the mouse cursor that has the same value for the mouse-face property. In your example, when the cursor is over the h, the whole text threefour gets highlighted since that's as far as the mouse-face property has the same value that it has for the h.

If you want to separate two buttons without having a visual separation, insert a zero-width space between them.

(progn (insert ?\n)
       (insert-text-button "one")
       (insert " ")
       (insert-text-button "two")
       (insert " ")
       (insert-text-button "three")
       (insert "\u200b")
       (insert-text-button "four"))