…ies#53283)

I was testing out the changes in
zed-industries#52730 and realized that the
agent panel, when full screen, would be auto-dismissed if I toggled the
sidebar off. Turns out this happens because we were "hard-coding" the
focus back to the center pane, which was automatically dismissing zoomed
items. So, in this PR, I essentially am copying the ModalLayer approach
of storing whatever was focused before so we can return focus back to it
if possible.

Release Notes:

- N/A

Follow up to #53283. That PR
replaced _where_ focus goes — from "always the center pane" to "the
saved element." But it didn't fix _when_ focus moves — it still moves
every time the sidebar closes, unconditionally. The problem is the saved
focus can be wrong... it's saved when the sidebar opens, but then it can
go to somewhere else after. An example:

1. Open sidebar → save focus (center pane), focus sidebar
2. Open agent panel from sidebar → focus moves to agent panel
3. Close sidebar → restore saved focus → **focuses center pane** (that's
what was saved in step 1, not the agent panel)

Effectively, this would still cause the agent panel to auto-dismiss.
This PR fixes it by no moving focus at all if the sidebar doesn't have
it. If the user already moved focus to the agent panel, there's nothing
to "restore".

Release Notes:

- N/A

…es#53386)

Follow up to zed-industries#53283. That PR
replaced _where_ focus goes — from "always the center pane" to "the
saved element." But it didn't fix _when_ focus moves — it still moves
every time the sidebar closes, unconditionally. The problem is the saved
focus can be wrong... it's saved when the sidebar opens, but then it can
go to somewhere else after. An example:

1. Open sidebar → save focus (center pane), focus sidebar
2. Open agent panel from sidebar → focus moves to agent panel
3. Close sidebar → restore saved focus → **focuses center pane** (that's
what was saved in step 1, not the agent panel)

Effectively, this would still cause the agent panel to auto-dismiss.
This PR fixes it by no moving focus at all if the sidebar doesn't have
it. If the user already moved focus to the agent panel, there's nothing
to "restore".

Release Notes:

- N/A

Follow up to #53283. That PR
replaced _where_ focus goes — from "always the center pane" to "the
saved element." But it didn't fix _when_ focus moves — it still moves
every time the sidebar closes, unconditionally. The problem is the saved
focus can be wrong... it's saved when the sidebar opens, but then it can
go to somewhere else after. An example:

1. Open sidebar → save focus (center pane), focus sidebar
2. Open agent panel from sidebar → focus moves to agent panel
3. Close sidebar → restore saved focus → **focuses center pane** (that's
what was saved in step 1, not the agent panel)

Effectively, this would still cause the agent panel to auto-dismiss.
This PR fixes it by no moving focus at all if the sidebar doesn't have
it. If the user already moved focus to the agent panel, there's nothing
to "restore".

Release Notes:

- N/A