Fix(gallery): Re-add image browsing with arrow keys

[DustyShoe]

Summary

This PR restores and hardens keyboard image browsing in Gallery/Viewer flows, including when panels are collapsed and when invoking from prompt-focused inputs.

Key Changes

1. Restored arrow-key browsing behavior

• Gallery arrow navigation works again in viewer-capable contexts.
• No longer requires clicking a gallery item first in common viewer flows.

2. Viewer-side Left/Right navigation when gallery is unavailable

• Left/Right now navigate prev/next image when:
  • fullscreen panel mode is active, or
  • right panel is collapsed, or
  • gallery panel is collapsed.
• Prevented handler conflicts by making grid hotkeys yield in viewer-arrow-navigation mode.

3. Paged Gallery cross-page keyboard navigation

• In paged mode, keyboard navigation indexes against full image list.
• Crossing page boundaries via arrows automatically changes page/selection correctly.

4. Shared prev/next navigation logic

• Extracted shared hook:
  • useNextPrevItemNavigation
• Both UI arrows (NextPrevItemButtons) and viewer hotkeys call the same prev/next logic.

5. Navigation API additions

• Added helpers in navigation-api:
  • isFullscreen(tab)
  • isRightPanelCollapsed(tab)
  • isGalleryPanelCollapsed(tab)
  • isViewerArrowNavigationMode(tab)

6. Invoke focus behavior (Generate + Upscaling)

• After invoke (button or hotkey), prompt/input focus is blurred and focus region is set to viewer in:
  • Generate
  • Upscaling
• Ensures arrow browsing is immediately available after invoke from text input.

Related Issues / Discussions

Fixes #8589

QA Instructions

A. Baseline Gallery Keyboard Nav

• Select image in visible gallery.
• Press Left/Right/Up/Down.
• Confirm selection moves as expected.

B. Viewer Without Extra Click

• Open viewer-capable tab.
• Do not click gallery item.
• Press arrows.
• Confirm browsing still works.

C. Right Panel Collapsed Only

• Collapse only right panel (G).
• Press Left/Right.
• Confirm prev/next image navigation works.

D. Fullscreen / Panels Hidden

• Toggle fullscreen panels mode (F).
• Press Left/Right.
• Confirm behavior matches viewer prev/next UI controls.

E. Paged Gallery Boundary

• Enable paged gallery view.
• Navigate to page boundary and press Left/Right.
• Confirm automatic jump to previous/next page selection.

F. Invoke From Prompt Input

• Focus prompt textarea in Generate, press Ctrl+Enter (Cmd+Enter on mac).
• Confirm input focus is cleared and arrow browsing works immediately.
• Repeat for Upscaling.
• Repeat with Ctrl+Shift+Enter / Cmd+Shift+Enter.

G. Input Safety

• Focus an input/textarea in contexts where viewer-arrow mode is not active.
• Confirm arrows behave as text-cursor keys (no unintended image nav).

Merge Plan

Should be simple merge.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[dunkeroni]

The structure here is good, and the feature works. However, I'm getting a flicker effect when switching between images that didn't happen in the old handling. The UI briefly flashes to a blank viewer before loading the new image. Seems to be related to how the refocusing is being done in focusViewerAfterInvoke.

[DustyShoe]

Updated this PR to keep the intended behavior while reducing regressions and code noise:

• fixed viewer flicker when switching images;
• centralized focus/blur handling in navigation-api and removed invoke-flow duplication;
• unified prev/next navigation via shared useNextPrevItemNavigation (UI buttons + hotkeys);
• removed UI noise where clicked next/prev buttons stayed visually highlighted on keyboard arrow use.

[lstein]

@dunkeroni Can you take a look at this and confirm that it works as expected? I'd like to get it into 6.12, maybe as early as this weekend.

[lstein]

I've put this through its paces and the system works smoothly. I don't see the flicker that @dunkeroni noted before the most recent commits.

I'm still looking for an official stamp of approval from @dunkeroni, but my opinion is that this PR is ready to merge.

[dunkeroni]

Sorry for letting this sit so long. Issues are resolved in my testing.

One thing to look out for (not necessarily unique to this PR): if you hold down an arrow key to run through a large board all at once, then it queues up all of the images to load and can take a while before the one you land on becomes visible. Can only really test that once and then you have to clear out the cache to see it happen again, otherwise all of them will load instantly. I don't think that is a scenario that most users will encounter outside of stress testing, but if we start supporting larger files that take longer to load (e.g. video) then we may need to implement a more elaborate loading queue.

[lstein]

Sorry for letting this sit so long. Issues are resolved in my testing.

One thing to look out for (not necessarily unique to this PR): if you hold down an arrow key to run through a large board all at once, then it queues up all of the images to load and can take a while before the one you land on becomes visible. Can only really test that once and then you have to clear out the cache to see it happen again, otherwise all of them will load instantly. I don't think that is a scenario that most users will encounter outside of stress testing, but if we start supporting larger files that take longer to load (e.g. video) then we may need to implement a more elaborate loading queue.

Yes, I encountered this too. Perhaps there should be some debouncing code added. Not a blocker to my mind.