1835421 - Print Preview dialog's light-to-dark-gray gradient background makes the scrollbar disappear at some position

Status: NEW

(Toolkit :: Printing, defect, P3)

Description

[Daniel Holbert [:dholbert]]

[Spinning this off as a followup to bug 1833733.]

The print preview dialog happens to use a gradient whose start and end colors are pretty similar to our "light" & "dark" overlay-scrollbar thumb colors. So whichever scrollbar color we choose, it's impossible to see at some position in the gradient (unless you actually hover the scroll track to make the scroll track itself show up).

Bug 1833733 [not yet landed] is making a correctness fix to our thumb-color-determination-logic, which happens to change our calculus and flip our scrollbar thumb to be light rather than dark. This makes it more visible when it's at the top, but unfortunately it's now impossible to see when mousewheel-scrolled all the way to the end, since the light-gray thumb blends into the light-gray background at the bottom of the gradient. Here's a screencast in a build with that bug's patch:
https://bug1833733.bmoattachments.org/attachment.cgi?id=9334909

I'm filing this bug as a followup to cover that remaining issue. Perhaps we should make the background a bit darker or a bit more consistent, so that a scrollbar that's visible at the top will also be visible at the bottom?

Comment 1

[Daniel Holbert [:dholbert]]

The background gradient is defined here:
https://searchfox.org/mozilla-central/rev/b891c79ab6bccbeba86ce7db11cbcb004f37d319/layout/style/res/ua.css#300-304

*|*::-moz-page-sequence {
  /* Collection of pages in print/print preview. Visual styles may only appear
   * in print preview. */
  display: block;
  background: linear-gradient(#606060, #8a8a8a) fixed;

Comment 2

[Daniel Holbert [:dholbert]]

Attachment: testcase 1 (mock-up of print-preview styling, after bug 1833733's patch)

Here's a reduced testcase to demonstrate how the print-preview scrollbar/background will look after bug 1833733 is fixed. (I've included a background-color #606060 behind the gradient, as we do in the patch for that bug, to help us determine a scrollbar color that's reasonable at the top of the page.)

If you scroll to the bottom of the page, with overlay scrollbars, the overlay scrollbar is impossible to see.

Comment 3

[Daniel Holbert [:dholbert]]

It's unfortunate that our scrollbar colors are such a near miss for the background color, in this neighborhood of gray shades for the background. I spun off bug 1835423 to see if there's any improvements we can make there.