Clients that exist on multiple tags are sometimes incorrectly displayed in the tag_preview widget

[gritting]

Problem Overview

If you view the tag_preview for a tag that contains client(s) on multiple tags, the tag_preview doesn't show correctly in some cases.

Demos

At 0:00, tag 1 has discord and watch nvidia-smi and tag 2 has cava and zsh. Everything works as expected at this point.
At 0:08, I add watch nvidia-smi to tag 2 (so it is on tag 1 and 2)
At 0:14, while on tag 2, I show that tag 2's tag_preview is correctly displayed. If the tag currently being viewed is the one you're attempting to view the tag_preview for (or was the last viewed tag which contains client(s) with multiple tags) then it displays correctly. (So hypothetically, if I were to switch to tag 3 at this point, tag 2's tag_preview would still display correctly.)
At 0:15, while still on tag 2, tag 1's watch nvidia-smi is completely missing from tag 1's tag_preview
At 0:19, I switch to tag 1, and show that tag 1's tag_preview is correct while viewing tag 1
At 0:21, while still on tag 1, I show that tag 2's tag_preview shows the content for watch nvidia-smi in the wrong place, obscuring the other clients in the tag_preview

tag_preview_weirdness.mp4

Unlike the example above, if the geometry and position of the client is exactly the same between its tags, it shows correctly in all tag_previews. You can see here, watch nvidia-smi is tiled on the left of tag 1 and 2, and displayed on the left side of both tag 1 and tag 2's tag_preview, as expected:

tag_preview_with_client_in_same_position.mp4

Another more severe example, with more clients. In this case, zenmonitor is the client on tags 4 and 7. You can see when on tag 4, viewing the tag_preview for tag 7, there is content from chromium, despite chromium not even being on tag 4 or 7, and content from zenmonitor is missing. The tag_preview is completely mangled too. Same problem when on tag 7 and viewing tag_preview for tag 4, the tag_preview is completely mangled:

tag_preview_example_2.mp4

Relevant configuration

The widget

bling.widget.tag_preview.enable {
    show_client_content = true,
    x = beautiful.useless_gap*2,
    y = beautiful.useless_gap*2 + beautiful.wibar_height,
    scale = 0.15,
    honor_padding = true,
    honor_workarea = false
}

The taglist is created with appropriate create_callback signals to show/hide the widget, all bog standard stuff

s.mytaglist = awful.widget.taglist {
        screen = s,
        filter = awful.widget.taglist.filter.noempty,
        buttons = taglist_buttons,
        style = { shape = function(cr,w,h) gears.shape.rounded_rect(cr,w,h,dpi(10)) end },
        widget_template = {
            id = "background_role",
            widget = wibox.container.background,
            forced_height = dpi(32),
            forced_width = dpi(32),
            {
                id = "text_role",
                align = "center",
                forced_width = dpi(32),
                forced_height = dpi(32),
                widget = wibox.widget.textbox
            },
            create_callback = function(self, c3, index, objects)
                self:connect_signal('mouse::enter', function()
                    local w = mouse.current_wibox
                    if w or old_taglist_entry then
                        taglist_cursor, old_taglist_entry = w.cursor, w
                        w.cursor = "hand2"
                    end
                    awesome.emit_signal("bling::tag_preview::update", c3)
                    awesome.emit_signal("bling::tag_preview::visibility", c3.screen, true)            
                end)

                self:connect_signal('button::press', function()
                    awesome.emit_signal("bling::tag_preview::visibility", c3.screen, false)
                end)

                self:connect_signal('mouse::leave', function()
                    if old_taglist_entry then
                        old_taglist_entry.cursor = taglist_cursor
                        old_taglist_entry = nil
                    end
                    awesome.emit_signal("bling::tag_preview::visibility", c3.screen, false)
                end)
            end
        }
    }

[javacafe01]

Thanks a lot for this bug report!