Drag scroll bar in a medium-sized string is too slow.

[tinchodias]

Evaluate this script, then drag the scroll bar a few seconds.

editor := AlbEditorElement new.
editor text: (String loremIpsum: 50000) asRopedText.

vscrollBar := BlVerticalScrollbarElement new.
vscrollBar constraintsDo: [ :c |
	c ignoreByLayout.
	c ignored horizontal alignRight ].
vscrollBar attachTo: editor infinite.

vscrollBar thumb
	when: BlDragStartEvent do: [ :evt |
		profiler := AndreasSystemProfiler new.
		profiler startProfiling ];
	when: BlDragEndEvent do: [ :evt |
		profiler stopProfiling.
		profiler doReport ].

space := BlSpace new.
space root addChild: editor.
space root addChild: vscrollBar.
space extent: 300@900.
space show.

This shows a profiler report in text, from where I focus on:

[[[                                                  68.4 (3,493)  AlbTextEditorLineSegmentHolder  pieceForElemen...ent:ifAbsent:
[[[                                                    67.4 (3,441)  AlbTextEditorLineSegmentPieceMap  pieceForElemen...ent:ifAbsent:
[[[                                                      56.800000000000004 (2,900)  Dictionary  associations
[[[                                                        |56.800000000000004 (2,900)  Array class  new:streamContents:
[[[                                                        |  43.5 (2,221)  WriteStream class [PositionableStream class]  on:
[[[                                                        |  12.5 (638)  Dictionary  associations
[[[                                                        |    12.4 (633)  Dictionary  associationsDo:
[[[                                                        |      11.8 (603)  Array [SequenceableCollection]  do:
[[[                                                        |        7.800000000000001 (398)  Dictionary  associationsDo:

68% of time is spent searching on the values of dictionaries (i.e. not searching a key, a scenario where Dictionaries perform better... it's searching among the values).

Even worse, all this calculation is part of answering AlbPrimarySelectionElement>>#selectionRectangles. When I logged the result, it was always an empty array!!!

So, 68% of time while dragging the scroll bar it is used in obtaining again and again an empty collection.