We should investigate the possibility to enhance the way we handle the hover effect on the message pane tree.

Problems

• Right now, it seems a row hover background colour happens only on Windows.
• After bug 1626841, the delete icon changes opacity when the entire row is hovered.
• It seems that the Tree element doesn't support detection of mouseover per column.

Possible solutions

• Add a light background hover effect for all the platforms.
• Increase the opacity of all the icons to mimic the delete icon behaviour...
• ...or drop any hover effect on the icons entirely.

We should investigate a proper and consistent solution to provide visual feedback when the user interacts with a row to prevent accidental clicks and improve the UX of that area.

This adds the hover state to all trees. Linux and Mac only as Windows has this already.

The hover state on a cell is not wanted when a row is hovered and the cell isn't. That's bad. There should be hover effect on a cell if the cell is hovered and is actionable, like a button. It will need code to set a pseudo selector based on mouse coordinates and based on whether a column is a cycler or pretending to be one. Barring that, hover effect on cells should be removed.

(In reply to Alessandro Castellani (:aleca) from comment #3)

Comment on attachment 9151572 [details] [diff] [review]
1638720-treechildren-hover.patch

Review of attachment 9151572 [details] [diff] [review]:

Looks quiet good, thanks.

How do you feel about a 0.1 background opacity instead of 0.15? The
current one seems a bit too prominent and I don't want to risk reducing the
readability contrast with the text.

I let it on 0.15 background opacity as 0.1 looks for me too faint. And Windows has already 0.15, see https://searchfox.org/mozilla-central/rev/ea7f70dac1c5fd18400f6d2a92679777d4b21492/toolkit/themes/shared/tree.inc.css#35.

Is there any chance of adding a 150ms transition for this background
change? With this we would have a consistent transition speed with our
toolbar buttons.

This doesn't work. Seems again that tree doesn't support such CSS.

As pointed out by alta88, please remove the hover effect I wrongly added on
the delete column.
This chunk can go:

treechildren::-moz-tree-image(deleteCol, hover),
treechildren::-moz-tree-image(deleteCol, selected),
treechildren::-moz-tree-image(deleteCol, imapdeleted) {
  opacity: 0.6;
}

Removed it and set the normal opacity 0.3 to 0.5. Okay or do you want 0.6?

(In reply to Alessandro Castellani (:aleca) from comment #5)

Too bad for the transition property of a tree item.

It really is. Just fyi, you also cannot use animated gifs in nsITree due to elevated cpu, even when they are no longer returned. There's a bug about it.

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/4b3e05877240
Implement the hover state for all tree rows on Linux and Mac. r=aleca