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I am having a rather unusual, intermittent issue with the Windows 7 taskbar. Based on everything I've found, both on SU and various Microsoft sites, part of the trouble is that several similar, but different, issues have been experienced by other users, who did not recognize the difference between their issues and this one, so they reported a successful resolution to their issue, which was not the same. Therefore, I will provide as much detail as possible, to distinguish my issue from others.

Note: while this question is very similar to Windows 7 taskbar thumbnails fail to appear, none of the answers on that question seem to be adequate. In particular, the accepted answer is not applicable, because it's a solution for all Aero transparency and visual effects not working - and even the OP commented on that answer that it didn't work (so I don't even know why it was accepted). This answer to the same questions, while definitely a high-quality answer, also does not appear to apply to this problem.

Typically, (assuming all Visual Effects options are enabled) when you place the mouse cursor over the taskbar icon for an open window, a thumbnail preview of that window appears just above the taskbar. If the window is one of several windows of the same program, Windows will also display previews of the other windows, in a horizontal row. Moving your mouse from the taskbar to one of these previews will make transparent all windows except the one represented by the preview (Aero Peek), and you can then click on the preview to make that window active, or right-click to access the system menu for that window (with options like Restore, Minimize, Maximize; right-clicking the taskbar icon itself brings up the system menu for the icon, with options like Pin/Unpin and an MRU list). If the mouse is moved away from the preview, without clicking, the previews disappear (as does any Peek you may have had visible).
If one program has many windows open, there is a threshold beyond which Windows will stop showing thumbnails, and will instead display a vertical text list of the open windows, but otherwise behave the same (e.g. still offers Aero Peek). This vertical list behavior is also executed when using the Aero Basic theme (but without Peek in that case).

In some circumstances, this behavior will suddenly stop functioning properly, and change to the following:
Placing the mouse over the taskbar icon causes it to illuminate, but no previews appear, nor even the Aero Basic-style vertical list. The icon can still be clicked to activate the window (if icons are not stacked), but if the mouse is moved off of the taskbar without clicking, the icon remains illuminated as if the mouse were still over it, and after a moment the window name will appear in a tooltip just above the icon (something that is only supposed to happen if the mouse remains over the taskbar icon). It's as if the taskbar fails to notice the mouse leaving its screen space and behaves as if the cursor were still in the last position it was in prior to moving off of it.

When the taskbar is in this state, the problem can usually be (very) temporarily resolved, by right-clicking on an icon for any open window in the taskbar - the menu appears without the MRU list at first, but then the MRU list appears after a second or so - then left-clicking on the icon (outside of the menu), which will minimize the window. Now, the previews are working, though they will usually stop working again within moments.

I am using the standard Aero Glass theme. Transparency works fine, so nothing is disabling Aero. Alt-Tab and Win-Tab both work fine, so it doesn't seem to be a problem with DWM. The Aero Troubleshooter doesn't detect any problems. All visual effects are turned on.

What's really causing this? Is there an actual fix (as opposed to a workaround like "end and restart the explorer.exe task", which may resolve one occurrence but doesn't prevent a recurrence), and/or a specific trigger that I can avoid?

It seems to happen more often when I have more programs running, but I'm not approaching 100% RAM utilization or anything. But I could believe that some particular program is a culprit. The only applications I regularly have open are:

  • Outlook 2010
  • Lync 2010
  • Word 2010
  • Excel 2010
  • Internet Explorer 11
  • Notepad
  • Task Manager
  • Lotus Notes 8.5
  • OneNote 2010 (notification icon only)
  • other notification icon utilities that came with the system

System is a Lenovo T430 running Windows 7 SP1.

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I have had similar experiences on machines, and have seen three things that have caused that type of effect:

  • The most common case was a huge number of 0 length files in the cache folders; hence, as soon as 4 or 5 programs opened, there were longer and longer delays as the cache folder filenames were processed; (this, of course, is also a function of drive speed/age).
  • A keyboard with a flaky ctrl key, that kept intermittently triggering; (that one was picked up when 4 computers all of a sudden started experiencing the same delays on a 4-port KVM).
  • A flaky USB port or mouse - always intermittent.

I have also noticed that effect more frequently if Internet Explorer is opened with a few windows, (lag time increases with the number of Internet Explorer panes opened), and some of those hyper-irritating video ads start silently downloading in the background (easy to check) if it happens, hit the esc key on each of the Internet Explorer panes, or pause any videos.

  • Well, it's definitely not an issue with the keyboard, mouse, or USB. I was intrigued by the cache files idea, but found no evidence of that in my case. I concur with your IE findings, but that doesn't really get us much closer to identifying the cause, nor any type of permanent solution. – Dan Henderson Jan 7 '16 at 22:19

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