Fixed: Topic 12380 - No image preview in list of blockable items

Modified fix from https://adblockplus.org/forum/viewtopic.php?f=11&t=12380#p85171

[Wladimir Palant, 2013-11-26 11:35:31]

http://codereview.adblockplus.org/6349471881363456/diff/5629499534213120/chro...
File chrome/content/ui/sidebar.js (right):

http://codereview.adblockplus.org/6349471881363456/diff/5629499534213120/chro...
chrome/content/ui/sidebar.js:307: let cacheListener = {
Style nit: this bracket should be on the next line.

http://codereview.adblockplus.org/6349471881363456/diff/5629499534213120/chro...
chrome/content/ui/sidebar.js:328: showPreview = false;
This should normally never happen, please add Cu.reportError(e) here to make
sure the error isn't swallowed.

http://codereview.adblockplus.org/6349471881363456/diff/5629499534213120/chro...
chrome/content/ui/sidebar.js:338: E("tooltipPreviewBox").hidden = true;
We should not show the preview box at all if we aren't sure that there will be a
preview. In other words, you should always do |E("tooltipPreviewBox").hidden =
true| first and only do |E("tooltipPreviewBox").hidden = false| and set image
src in onCacheEntryAvailable.

[Wladimir Palant, 2013-11-26 11:45:25]

Btw, I checked why the current code assigns an empty string to src only to
overwrite it with the actual URL one line below - this goes back to revision 191
from March 2006 when image previews were originally introduced, no obvious
reason why it was done this way. In other words, this seems to be a very old
bug.

[Thomas Greiner, 2013-11-29 14:35:44]

http://codereview.adblockplus.org/6349471881363456/diff/5629499534213120/chro...
File chrome/content/ui/sidebar.js (right):

http://codereview.adblockplus.org/6349471881363456/diff/5629499534213120/chro...
chrome/content/ui/sidebar.js:307: let cacheListener = {
On 2013/11/26 11:35:31, Wladimir Palant wrote:
> Style nit: this bracket should be on the next line.

Done.

http://codereview.adblockplus.org/6349471881363456/diff/5629499534213120/chro...
chrome/content/ui/sidebar.js:328: showPreview = false;
On 2013/11/26 11:35:31, Wladimir Palant wrote:
> This should normally never happen, please add Cu.reportError(e) here to make
> sure the error isn't swallowed.

Done.

http://codereview.adblockplus.org/6349471881363456/diff/5629499534213120/chro...
chrome/content/ui/sidebar.js:338: E("tooltipPreviewBox").hidden = true;
On 2013/11/26 11:35:31, Wladimir Palant wrote:
> We should not show the preview box at all if we aren't sure that there will be
a
> preview. In other words, you should always do |E("tooltipPreviewBox").hidden =
> true| first and only do |E("tooltipPreviewBox").hidden = false| and set image
> src in onCacheEntryAvailable.

Done.

[Wladimir Palant, 2013-12-02 07:36:58]

LGTM