fix Replace preview

[w-e-w]

Description

• fix bug [Bug]: Replace preview button always saves gird image #16115

• the cause is Always show extra networks tabs in the UI #11808 when the extra networks are moved to tabs

1. when an extra networks tab is selected, the generation tabs is deselected
2. when a tab is deselected the tab and child elements on the tab such as gallery is not longer displayed
3. when something is not displayed the .offsetParent property is null

in all_gallery_buttons() we use the when not displayed offsetParent is null behavior to select only the gallery that is currently being displayed

stable-diffusion-webui/javascript/ui.js

Line 14 in a30b19d

if (elem.parentElement.offsetParent) {

but this no longer works after #11808 as the tab gallery will always be not displayed when we switched to the extra networks tab to replace the preview

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fix solution
Implement a different galleries selector base on element ID to directly to select the specific gallery

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I create a new function because I don't want to touch the existing selected_gallery_index() in case it breaks something else

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests