Add new style sheet only if it differs from old style sheet

[mjethani]

Background

In #5864 we started maintaining only one style sheet for element hiding emulation. The changes for this add the new style sheet even if it is identical to the old style sheet (and then remove the old style sheet if it differs), as explained in this comment:

// Ideally we would compare the old and new style sheets and skip this code
// if they're the same, but the old style sheet can be a leftover from a
// previous instance of the frame. We must add the new style sheet
// regardless.

Two things have changed since:

1. We no longer use style sheets for element hiding emulation (#6504)
2. The changes for #7104 should already ensure that the old style sheet is deleted from the frame object when the frame's contents have changed

It should be possible now to simplify the code so that the style sheet is updated only if it has really changed.

What to change

[To be determined]

[jha2125]

spam

[sebastian]

Sorry, but we switched to GitLab. If this issue is still relevant, please file it again in the ​new issue tracker.