macOS 14.7.2
Illustrator ver.29.7.1(probably, up to 29.6.1)

再現手順
1. アートボード上にオブジェクトを配置し、その上に不透明マスクとなるオブジェクトを配置する。
2. 2つのオブジェクトを選択した状態でプロパティパネルの「アピアランス」にある「不透明度」をクリックする。
3. ポップアップした透明ダイアログ上の「マスクを作成」をクリックすると不透明マスクが適用される。
4. ポップアップ上のマスク側のプレビューをクリックするとアートボード上のオブジェクトはマスク側オブジェクトが選択される。
5. マスクを編集するためにオブジェクトをクリックするとマスクされたオブジェクトの選択に切り替わりマスクが編集できない。
6. マスク編集状態からプロパティパネル経由でマスク編集を終了しようとしてもマスク編集に戻ってしまう。

期待する結果
プロパティパネルの透明ポップアップパネルの操作でオブジェクトとマスクの編集が正常に切り替わる。

実際の結果
プロパティパネルの透明ポップアップパネルではオブジェクトとマスクの切り替えが出来ない。

現状の回避方法
透明パネルを利用する。

ここではプロパティパネルについて報告していますが、上部のコントロールバーの「不透明度」のポップアップも同じ挙動です。

EN
Step to Reproduce
1. place an object on the artboard, and place an object that will be the opacity mask on top of it.
2. with the two objects selected, click "Opacity" under "Appearance" in the Properties panel.
3. click "Create Mask" on the transparency dialog that pops up to apply the opacity mask.
4. Clicking the mask-side preview on the pop-up selects the mask-side object.
5. When we click on an object to edit the mask, it switches to selecting the masked object and the mask cannot be edited.
6. when we try to exit mask editing via the properties panel from the mask editing state, it returns to the mask editing state.

Expected Result
The operation of the transparent pop-up panel in the properties panel switches between object and mask editing normally.

Actual Result
Opacity popup panel in the Properties panel does not switch between objects and masks.

Current workaround
Use the transparency panel.

Reporting here about the Properties panel, but the "Opacity" popup on the top "Control Bar" has the same behavior.
This bug was reported in the following thread

https://community.adobe.com/t5/illustratorフォーラム-discussions/オプションバーの-透明-のマスク編集で-通常編集画面に復帰できない/td-p/15479971?profile.language=ja

Thank you very much Egor, this helps, good stuff! Adobe really should fix the one in the context panel though. I was used to using that one and it worked great for many years. Users expect software updates to improve functionality, not make things worse. Thanks again

Please let me know when this issue will be resolved. What has Adobe been doing from May to July? It's truly pointless to update and cram in new features while important older features with bugs remain unfixed.

Here’s a brief demonstration.
The mask is not created at all or kicks you out of it when Properties or Control panels are used.
No problem with the standalone 'true' Transparency panel.
'Dropdown panels' see the mask switching only when we also toggle an option back and forth.

The pop-up window in the Properties has a problem with the Transparency panel because it does not exit the mask mode. The panel must be opened separately.
Then, when we are in the mask mode, there is no special mode.

I would like to make the Properties panel more free, for example, open the full aperture.

Can confirm this is still happening as of 7/4/2025 in Illustrator 29.6.1 for Windows. It only works if I first uncheck and then recheck the Clip option, or if I use the Window > Transparency standalone panel.

I'm facing the same issue. I can go art to mask or mask to art when I click on Opacity

Just the Standalone Transparency Panel is working fine to me.

Since updating to Adobe Illustrator version 29.5.1 (64-bit) on Windows 11, there is a critical bug in the Transparency panel’s opacity mask behavior:

When I select the object thumbnail inside the Transparency panel, Illustrator immediately switches the selection to the opacity mask thumbnail when I exit or deselect.

When I select the opacity mask thumbnail, Illustrator switches back to the object thumbnail after I exit the panel or click outside.

The selection keeps flipping back and forth automatically, making it nearly impossible to edit either the object or the mask.

This was not an issue in earlier Illustrator versions. In prior versions, Illustrator would maintain the selected object or mask until I manually changed it. This regression in 29.5.1 breaks normal opacity mask editing workflows.

Expected behavior:

Illustrator should maintain the user’s current thumbnail selection (object or mask) inside the Transparency panel, even after clicking outside or switching focus, until the user manually selects a different thumbnail.

Illustrator version: 29.5.1 (64-bit)
OS version: Windows 11

I reinstalled my old version of Illustrator and it works perfectly as I always wanted. (Version 29.4) Please check the bug again and I hope it will be fixed in the next update. I am fine with the old version now.

@Công, I see. You use the dropdown form the Control panel specifically.
I’ve merged your report with an existing one.
1. Does the standalone Opacity panel work fine?
2. Does toggling Clip back and forth helps to enter the opacity mask if you use Control panel’s Opacity?

Here you are

@Công, it works when I try... take a look at the GIF attached.
Please record a video or a GIF to show what happens for you when you try.

Can't create and edit opacity mask in Illustrator. When I click to double click to opacity mask and creat a shape. This function does not work

Laurence, I confirm this.
It works for me fine when I try to do the same using the standalone Transparency panel — does it work for you too?