taskmanager: Fix flickering when scrolling in GroupDialog
Commit 1
In updateMaxTextWidth(), maxTextWidth was reset to 0 and was updated whenever a new maximum value was found, which caused flickering when scrolling in a GroupDialog.
Use a temporary value to store the result, and only update maxTextWidth in the end.
Commit 2
Because updateMaxTextWidth() only considers the first 20 items, enable
onLabelTextChanged only for the first 20 items.
Commit 3
The hack makes scrollbar visible when it should not be visible because of binding loops in ScrollView.
Edited by Fushan Wen