Fill free space on Panel bugs

[Covkie]

Title vanishes while hovered with a spacer on the same panel + other issues with spacers on the same panel. Prevents a use case of application-title-bar, global menu, spacer, other widgets from working:

in the edit mode you can see that the title has vanished

After exiting the title area is not clickable/draggable and only the icon display allows interaction

Fill free space on Panel is limited to the Maximum width parameter (currently have it set to 2000 to workaround this). When set to a value less than the free space the taken space changes when hovering with the Show disabled elements Hide option

Filling free space is attached to the "Title" element so removing it or it having no contents breaks the fill free space (Unrelated to the issue title but related to this part "Mouse area drag" is also attached to the title rather than blank widget space

[antroids]

Title vanishes while hovered with a spacer on the same panel + other issues with spacers on the same panel. Prevents a use case of application-title-bar, global menu, spacer, other widgets from working:
After exiting the title area is not clickable/draggable and only the icon display allows interaction

Could you check the 0.6.9
I've added preferred size property, it should be used to calculate panel layout with spacers.

Fill free space on Panel is limited to the Maximum width parameter (currently have it set to 2000 to workaround this). When set to a value less than the free space the taken space changes when hovering with the Show disabled elements Hide option

I hope this problem will not be relevant anymore.
Maximim/Minimum width parameters are for Title.

Filling free space is attached to the "Title" element so removing it or it having no contents breaks the fill free space (Unrelated to the issue title but related to this part "Mouse area drag" is also attached to the title rather than blank widget space

I was unable to create several overlapped mouse handlers for buttons and widget background.
But you still can use a space as Title Undefined value.

[Covkie]

Aug 06 01:04:23 bakery plasmashell[5577]: file:///home/kylie/.local/share/plasma/plasmoids/com.github.antroids.application-title-bar/contents/ui/main.qml:326:9: QML RowLayout: RowLayout called polish() inside updatePolish() of RowLayout
Aug 06 01:04:23 bakery plasmashell[5577]: file:///home/kylie/.local/share/plasma/plasmoids/com.github.antroids.application-title-bar/contents/ui/main.qml:326:9: QML RowLayout: possible QQuickItem::polish() loop

Plasmashell crashes with this spamming on 7.0.0 - panels weren't spawning and couldn't interact with the desktop at all. Well and truly exploded.

In the time before the crash (I toggled "Fill free space on panel") I couldn't see the preferred size property option

[antroids]

Plasmashell crashes with this spamming on 7.0.0 - panels weren't spawning and couldn't interact with the desktop at all. Well and truly exploded.

Strange, I'm using it for several days without any problems. You usually need to kill plasmashell in such cases.
Could you export the config of your widget via context menu -> Export Configuration and attach it.
The version 0.6.8 can be reverted with:
wget https://github.com/antroids/application-title-bar/releases/download/v0.6.8/application-title-bar.plasmoid -O ${TMPDIR:-/tmp}/application-title-bar.plasmoid && kpackagetool6 -t Plasma/Applet -u ${TMPDIR:-/tmp}/application-title-bar.plasmoid && systemctl --user restart plasma-plasmashell.service

In the time before the crash (I toggled "Fill free space on panel") I couldn't see the preferred size property option

It is enabled when Fill free space is active, you don't need to do anything.

UPD: I have tried to reproduce the issue but with no luck so far. Could you also attach your QT version from "About this System".